[Docs] Refactor docs

[zhouzaida]

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Motivation

Refactor the documentation. Documentation can be visited at https://mmcv-refactor-docs.readthedocs.io/en/latest/

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

[codecov]

Codecov Report

Merging #1102 (47386dd) into master (76d9bf1) will increase coverage by 0.03%.
The diff coverage is 44.44%.

❗ Current head 47386dd differs from pull request most recent head 88f3060. Consider uploading reports for the commit 88f3060 to get more accurate results

@@            Coverage Diff             @@
##           master    #1102      +/-   ##
==========================================
+ Coverage   67.96%   67.99%   +0.03%     
==========================================
  Files         159      159              
  Lines       10416    10426      +10     
  Branches     1896     1897       +1     
==========================================
+ Hits         7079     7089      +10     
+ Misses       2969     2968       -1     
- Partials      368      369       +1     

Flag	Coverage Δ
unittests	67.99% <44.44%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcv/parallel/distributed.py	22.44% <20.00%> (+1.61%)	⬆️
mmcv/ops/saconv.py	83.58% <25.00%> (-4.95%)	⬇️
mmcv/runner/fp16_utils.py	58.38% <25.00%> (+0.26%)	⬆️
mmcv/cnn/bricks/activation.py	88.00% <50.00%> (+0.50%)	⬆️
mmcv/parallel/distributed_deprecated.py	67.39% <50.00%> (+0.72%)	⬆️
mmcv/runner/dist_utils.py	50.49% <50.00%> (+0.49%)	⬆️
mmcv/runner/hooks/logger/tensorboard.py	36.36% <50.00%> (+1.98%)	⬆️
mmcv/runner/hooks/optimizer.py	17.29% <50.00%> (+0.62%)	⬆️
mmcv/runner/epoch_based_runner.py	83.80% <66.66%> (+0.95%)	⬆️
mmcv/runner/iter_based_runner.py	57.85% <66.66%> (+0.71%)	⬆️
... and 1 more

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[ZwwWayne]

Downstream OpenMMLab projects all rely on the CONTRIBUTING.md at https://github.com/open-mmlab/mmcv/blob/master/CONTRIBUTING.md