remove mentioning external tex files from contribution guide

.github/CONTRIBUTING.md

@@ -73,7 +73,7 @@ Here is a checklist of steps you need to follow to submit a single file or user
 * If you want your contribution to be added as a user-contributed feature, and it is a single file (actually a `<name>.cpp` and `<name>.h` file) it can be rapidly added to the USER-MISC directory. Include the one-line entry to add to the USER-MISC/README file in that directory, along with the 2 source files. You can do this multiple times if you wish to contribute several individual features.
 * If you want your contribution to be added as a user-contribution and it is several related features, it is probably best to make it a user package directory with a name like USER-FOO. In addition to your new files, the directory should contain a README text file. The README should contain your name and contact information and a brief description of what your new package does. If your files depend on other LAMMPS style files also being installed (e.g. because your file is a derived class from the other LAMMPS class), then an Install.sh file is also needed to check for those dependencies. See other README and Install.sh files in other USER directories as examples. Send us a tarball of this USER-FOO directory.
 * Your new source files need to have the LAMMPS copyright, GPL notice, and your name and email address at the top, like other user-contributed LAMMPS source files. They need to create a class that is inside the LAMMPS namespace. If the file is for one of the USER packages, including USER-MISC, then we are not as picky about the coding style (see above). I.e. the files do not need to be in the same stylistic format and syntax as other LAMMPS files, though that would be nice for developers as well as users who try to read your code.
-* You **must** also create or extend a documentation file for each new command or style you are adding to LAMMPS. For simplicity and convenience, the documentation of groups of closely related commands or styles may be combined into a single file. This will be one file for a single-file feature. For a package, it might be several files. These are files in the [reStructuredText](https://docutils.sourceforge.io/rst.html) markup language, that are then converted to HTML and PDF. The tools for this conversion are included in the source distribution, and the translation can be as simple as doing "make html pdf" in the doc folder. Thus the documentation source files must be in the same format and style as other `<name>.rst` files in the lammps/doc/src directory for similar commands and styles; use one or more of them as a starting point. An introduction to reStructuredText can be found at [https://docutils.sourceforge.io/docs/user/rst/quickstart.html](https://docutils.sourceforge.io/docs/user/rst/quickstart.html). As appropriate, the text files can include mathematical expressions in MathJAX markup or links to equations (see doc/Eqs/*.tex for examples, we auto-create the associated JPG files), or figures (see doc/JPG for examples), or even additional PDF files with further details (see doc/PDF for examples). The doc page should also include literature citations as appropriate; see the bottom of doc/fix_nh.rst for examples and the earlier part of the same file for how to format the cite itself. The "Restrictions" section of the doc page should indicate that your command is only available if LAMMPS is built with the appropriate USER-MISC or USER-FOO package. See other user package doc files for examples of how to do this. The prerequisite for building the HTML format files are Python 3.x and virtualenv. Please run at least `make html` and `make spelling` and carefully inspect and proofread the resulting HTML format doc page as well as the output produced to the screen. Make sure that all spelling errors are fixed or the necessary false positives are added to the `doc/utils/sphinx-config/false_positives.txt` file.  For new styles, those usually also need to be added to lists on the respective overview pages. This can be checked for also with `make style_check`.
+* You **must** also create or extend a documentation file for each new command or style you are adding to LAMMPS. For simplicity and convenience, the documentation of groups of closely related commands or styles may be combined into a single file. This will be one file for a single-file feature. For a package, it might be several files. These are files in the [reStructuredText](https://docutils.sourceforge.io/rst.html) markup language, that are then converted to HTML and PDF. The tools for this conversion are included in the source distribution, and the translation can be as simple as doing "make html pdf" in the doc folder. Thus the documentation source files must be in the same format and style as other `<name>.rst` files in the lammps/doc/src directory for similar commands and styles; use one or more of them as a starting point. An introduction to reStructuredText can be found at [https://docutils.sourceforge.io/docs/user/rst/quickstart.html](https://docutils.sourceforge.io/docs/user/rst/quickstart.html). The text files can include mathematical expressions and symbol in ".. math::" sections or ":math:" expressions or figures (see doc/JPG for examples), or even additional PDF files with further details (see doc/PDF for examples). The doc page should also include literature citations as appropriate; see the bottom of doc/fix_nh.rst for examples and the earlier part of the same file for how to format the cite itself. The "Restrictions" section of the doc page should indicate that your command is only available if LAMMPS is built with the appropriate USER-MISC or USER-FOO package. See other user package doc files for examples of how to do this. The prerequisite for building the HTML format files are Python 3.x and virtualenv. Please run at least `make html`, `make pdf` and `make spelling` and carefully inspect and proofread the resulting HTML format doc page as well as the output produced to the screen. Make sure that all spelling errors are fixed or the necessary false positives are added to the `doc/utils/sphinx-config/false_positives.txt` file.  For new styles, those usually also need to be added to lists on the respective overview pages. This can be checked for also with `make style_check`.
 * For a new package (or even a single command) you should include one or more example scripts demonstrating its use. These should run in no more than a couple minutes, even on a single processor, and not require large data files as input. See directories under examples/USER for examples of input scripts other users provided for their packages. These example inputs are also required for validating memory accesses and testing for memory leaks with valgrind
 * If there is a paper of yours describing your feature (either the algorithm/science behind the feature itself, or its initial usage, or its implementation in LAMMPS), you can add the citation to the *.cpp source file. See src/USER-EFF/atom_vec_electron.cpp for an example. A LaTeX citation is stored in a variable at the top of the file and a single line of code that references the variable is added to the constructor of the class. Whenever a user invokes your feature from their input script, this will cause LAMMPS to output the citation to a log.cite file and prompt the user to examine the file. Note that you should only use this for a paper you or your group authored. E.g. adding a cite in the code for a paper by Nose and Hoover if you write a fix that implements their integrator is not the intended usage. That kind of citation should just be in the doc page you provide.