[Sphinx] Adjust for source_parsers deprecation in Sphinx 3.0

Update the Sphinx configuration for the removal of source_parsers in
Sphinx 3.0. The variable has been deprecated since version 1.8.

> Version 1.8 deprecates and version 3.0 removes the source_parsers
> configuration variable that was used by older recommonmark versions.

https://www.sphinx-doc.org/en/master/usage/markdown.html

Differential revision: https://reviews.llvm.org/D75284

llvm/docs/conf.py

@@ -26,15 +26,16 @@ from datetime import date
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ['sphinx.ext.intersphinx', 'sphinx.ext.todo']
+extensions = ['sphinx.ext.intersphinx', 'sphinx.ext.todo', 'recommonmark']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
 
 # The suffix of source filenames.
-source_suffix = ['.rst', '.md']
-
-source_parsers = {'.md': 'recommonmark.parser.CommonMarkParser'}
+source_suffix = {
+    '.rst': 'restructuredtext',
+    '.md': 'markdown',
+}
 
 # The encoding of source files.
 #source_encoding = 'utf-8-sig'