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/README

@@ -0,0 +1,37 @@
+# How to build Sphinx documentation
+
+In order to build documentation for the standard library, first install sphinx. Run these commands in the `gem5` directory.
+
+```bash
+python3 -m venv build/
+source build/bin/activate
+pip install sphinx
+
+```
+
+Next, build gem5, then cd to the `docs` directory.
+
+```bash
+scons build/ALL/gem5.opt
+cd docs
+```
+
+In the `docs` directory, run `../build/ALL/gem5.opt gem5-sphinx-apidoc -o . ../src/python/gem5 -F -e` in order to generate the RST files.
+
+To generate the documentation, run `SPHINXBUILD="../build/ALL/gem5.opt gem5-sphinx" make html`
+
+In order to view this documentation as a website, run the following commands while still in the `docs` directory:
+
+```bash
+cd _build/html
+python3 -m http.server 8000
+```
+
+If you want to use a different Sphinx theme, follow these steps:
+
+- Install the desired theme, e.g., `pip install sphinx-rtd-theme`
+- Update the `html_theme` in `docs/conf.py` by setting `html_theme = "sphinx_rtd_theme"`
+- Run `SPHINXBUILD="../build/ALL/gem5.opt gem5-sphinx" make html` in the `docs` directory to apply the changes.
+
+Note: We are aware that some modules currently display a blank page in the documentation.
+This will be addressed in the future.