Add markdown lint in pre-commit hook

[xvjiarui]

No description provided.

[codecov]

Codecov Report

Merging #4255 (1372c6f) into master (bec3fc1) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4255      +/-   ##
==========================================
- Coverage   62.69%   62.58%   -0.12%     
==========================================
  Files         232      232              
  Lines       17603    17603              
  Branches     2945     2945              
==========================================
- Hits        11037    11017      -20     
- Misses       6027     6049      +22     
+ Partials      539      537       -2     

Flag	Coverage Δ
unittests	62.55% <ø> (-0.11%)	⬇️

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Impacted Files	Coverage Δ
mmdet/models/roi_heads/test_mixins.py	45.73% <0.00%> (-8.53%)	⬇️
mmdet/models/detectors/cornernet.py	94.87% <0.00%> (-5.13%)	⬇️
mmdet/models/dense_heads/corner_head.py	74.86% <0.00%> (-1.92%)	⬇️

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