Docs don't build under the Python 3 TF2 branch

[mkgkruse]

When building docs via "make html" sphinx returns an error regarding a theme.

make html

Output from command line:

sphinx-build -b html -d _build/doctrees . _build/html
Running Sphinx v3.5.3
making output directory... done
WARNING: html_static_path entry '_static' does not exist

Theme error:
sphinx_rtd_theme is no longer a hard dependency since version 1.4.0. Please install it manually.(pip install sphinx_rtd_theme)
make: *** [html] Error 2

After pip installing theme
Success

[lucpeterson]

The fix should just be to add sphinx_rtd_theme to the requirements.txt