[Docs] Output pdf and epub formats for readthedocs

[zhouzaida]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Support multi formats of readthedocs

Modification

.readthedocs.yml

BC-breaking (Optional)

No

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

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

Merging #1237 (ea38b48) into master (285a052) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1237   +/-   ##
=======================================
  Coverage   68.27%   68.27%           
=======================================
  Files         160      160           
  Lines       10599    10599           
  Branches     1937     1937           
=======================================
  Hits         7236     7236           
  Misses       2979     2979           
  Partials      384      384           

Flag	Coverage Δ
unittests	68.27% <ø> (ø)

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