[Docs] Temporarily skip errors in building pdf docs at readthedocs

[C1rN09]

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Motivation

PR #915 spoiled docs building process in readthedocs. Reasons under investigation. This PR temporarily disabled building pdf docs to skip this error.

Modification

BC-breaking (Optional)

Use cases (Optional)

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@502edec). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head 6e8cf89 differs from pull request most recent head 8ec7e26. Consider uploading reports for the commit 8ec7e26 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #928   +/-   ##
=======================================
  Coverage        ?   77.92%           
=======================================
  Files           ?      133           
  Lines           ?    10097           
  Branches        ?     2011           
=======================================
  Hits            ?     7868           
  Misses          ?     1888           
  Partials        ?      341           

Flag	Coverage Δ
unittests	77.92% <0.00%> (?)

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