[Improvement] Lint markdown files

[dreamerlin]

This PR adds markdown lint in pre-commit hook, what a heavy job !

[codecov]

Codecov Report

Merging #225 (3cf1c3f) into master (f4b165b) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
+ Coverage   84.69%   84.73%   +0.03%     
==========================================
  Files         118      118              
  Lines        8344     8344              
  Branches     1366     1366              
==========================================
+ Hits         7067     7070       +3     
+ Misses        930      929       -1     
+ Partials      347      345       -2     

Flag	Coverage Δ
unittests	84.71% <ø> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
mmaction/core/evaluation/accuracy.py	94.52% <0.00%> (+1.36%)	⬆️

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