misc: Add sphinx stdlib documentation (#335)

This PR adds documentation to the standard library using Sphinx. For
details on how the documentation was generated, refer to
https://gem5.atlassian.net/browse/GEM5-1314. Currently, some modules
like `dramsys` and `mesi_three_level` appear as blank pages. To view the
current state of the documentation locally, run: `cd docs/_build/html;
python3 -m http.server 8000`


---------
Co-authored-by: ivanaamit <ivanamit91@gmail.com>

docs/gem5-sphinx-apidoc

@@ -0,0 +1,14 @@
+import re
+import sys
+
+extensions = ['myst_parser']
+
+source_suffix = {
+    '.md': 'markdown',
+    '.py': 'markdown'
+}
+
+from sphinx.ext.apidoc import main
+if __name__ == '__m5_main__':
+    sys.argv[0] = re.sub(r'(-script\.pyw|\.exe)?$', '', sys.argv[0])
+    sys.exit(main())