Add link checking for compiled docs

[anishathalye]

Example failing run: https://github.com/anishathalye/cleanlab/runs/5870279217?check_suite_focus=true (reST parsed this link incorrectly)
Example passing run: https://github.com/anishathalye/cleanlab/runs/5870656641?check_suite_focus=true

[codecov]

Codecov Report

Merging #188 (8de8340) into master (f5a0ca5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #188   +/-   ##
=======================================
  Coverage   95.68%   95.68%           
=======================================
  Files          10       10           
  Lines         812      812           
  Branches      175      175           
=======================================
  Hits          777      777           
  Misses         12       12           
  Partials       23       23           

Impacted Files	Coverage Δ
cleanlab/internal/label_quality_utils.py	100.00% <ø> (ø)

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[jwmueller]

Super useful!