Redirect RTD site to docs.cleanlab.ai: Part 2

[weijinglok]

This PR is a follow-up to #130. https://cleanlab.readthedocs.io now redirects to https://docs.cleanlab.ai. Admin will need to perform step 4 below before merging this PR.

• 1. Add "Redirects" to the cleanlab project in RTD
• 2. Add the index.rst (redirecting HTML) and change Python to v3.8 in .readthedocs.yaml in the cleanlab's master branch
• 3. Allow RTD to build this latest version (AKA master branch) of the docs and disable any other versions except latest
• 4. Admin to disable the RTD webhook from https://github.com/cleanlab/cleanlab/settings/hooks
• 5. Delete the index.rst and .readthedocs.yaml files in the cleanlab's master branch

[codecov]

Codecov Report

Merging #132 (e8be62e) into master (5203493) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #132   +/-   ##
=======================================
  Coverage   86.89%   86.89%           
=======================================
  Files          11       11           
  Lines         908      908           
  Branches      166      166           
=======================================
  Hits          789      789           
  Misses        103      103           
  Partials       16       16           

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[jwmueller]

I've disabled the RTD webhook from https://github.com/cleanlab/cleanlab/settings/hooks