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