[Enhance] update building docs

[Xiangxu-0103]

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Motivation

Reference: https://github.com/open-mmlab/mmocr/blob/dev-1.x/docs/en/conf.py

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

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.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (aa10365) 46.99% compared to head (dfcea44) 46.99%.

❗ Current head dfcea44 differs from pull request most recent head dfc2f52. Consider uploading reports for the commit dfc2f52 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #2419   +/-   ##
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  Coverage    46.99%   46.99%           
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  Files          275      275           
  Lines        22904    22904           
  Branches      3566     3566           
========================================
  Hits         10763    10763           
  Misses       11455    11455           
  Partials       686      686           

Flag	Coverage Δ
unittests	46.99% <ø> (ø)

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[JingweiZhang12]

Please check the influence of all modifications. The examples of mmdet may not be suitable, For example, we'd better keep the links of 'upstream' in readthedocs.