Fix Sphinx bugs to enable readthedocs

[MooreZheng]

What type of PR is this?
/kind bug

What this PR does / why we need it:
There are configuration and package-absent errors that are fixed with Sphinx codes in this PR.

1. Add readthedocs.yaml to enable Sphinx, which is necessary for building readthedocs projects. References are as https://stackoverflow.com/questions/60364789/how-do-i-correctly-start-using-readthedocs-yml and https://docs.readthedocs.io/en/stable/config-file/index.html

2. Add the try/except install session, add the extension and import the package for sphinx_rtd_theme. That is because sphinx_rtd_theme is no longer a hard dependency since version 1.4.0

Which issue(s) this PR fixes:
Related to #145 and #92

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