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/Makefile

@@ -0,0 +1,20 @@
+# Minimal makefile for Sphinx documentation
+#
+
+# You can set these variables from the command line, and also
+# from the environment for the first two.
+SPHINXOPTS    ?=
+SPHINXBUILD   ?= sphinx-build
+SOURCEDIR     = .
+BUILDDIR      = _build
+
+# Put it first so that "make" without argument is like "make help".
+help:
+	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+
+.PHONY: help Makefile
+
+# Catch-all target: route all unknown targets to Sphinx using the new
+# "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
+%: Makefile
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)