[doc] added section on generating the html doc

Added a new section on generating the html documentation
from the rst/md sources to our documentation.

Background: I wanted to check what my documenation
changes would look like on the website and had a hard
time finding how to do that. So I wanted to save other
folks the effort.

Differential Revision: https://reviews.llvm.org/D107460

llvm/docs/MarkdownQuickstartTemplate.md

@@ -155,3 +155,7 @@ integration documentation can be found in the [recommonmark docs].
 
 [Common Mark spec]: http://spec.commonmark.org/0.28/
 [recommonmark docs]: http://recommonmark.readthedocs.io/en/latest/index.html
+
+## Generating the documentation
+
+see [Sphinx Quickstart Template](SphinxQuickstartTemplate.html#generating-the-documentation)

llvm/docs/SphinxQuickstartTemplate.rst

@@ -165,3 +165,32 @@ without any syntax highlighting like this:
                           ...
 
 
+Generating the documentation
+============================
+
+You can generate the HTML documentation from the sources locally if you want to
+see what they would look like. In addition to the normal 
+`build tools <docs/GettingStarted.html>`_
+you need to install `Sphinx`_ and the 
+`recommonmark <https://recommonmark.readthedocs.io/en/latest/>`_ extension.
+
+On Debian you can install these with:
+
+.. code-block:: console
+
+   sudo apt install -y sphinx-doc python-recommonmark-doc
+
+Then run cmake to build the documentation inside the ``llvm-project`` checkout:
+
+.. code-block:: console
+
+   mkdir build
+   cd build
+   cmake -DLLVM_ENABLE_SPHINX=On ../llvm
+   cmake --build . --target docs-llvm-html
+
+In case you already have the Cmake build set up and want to reuse that, 
+just set the CMake variable ``LLVM_ENABLE_SPHINX=On``.
+
+After that you find the generated documentation in ``build/docs/html``
+folder.