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 https://github.com/kubeedge/ianvs/issues/145 and https://github.com/kubeedge/ianvs/issues/92

[kubeedge-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[MooreZheng]

/approve