[Docs] Replace markdownlint with mdformat for avoiding installing ruby

[zytx121]

related pr open-mmlab/mmcv#1936

[codecov]

Codecov Report

Merging #306 (606b431) into dev (d11241f) will increase coverage by 3.20%.
The diff coverage is 51.19%.

@@            Coverage Diff             @@
##              dev     #306      +/-   ##
==========================================
+ Coverage   28.30%   31.51%   +3.20%     
==========================================
  Files         108      113       +5     
  Lines        6967     7346     +379     
  Branches     1048     1126      +78     
==========================================
+ Hits         1972     2315     +343     
- Misses       4927     4929       +2     
- Partials       68      102      +34     

Flag	Coverage Δ
unittests	31.48% <50.95%> (+3.19%)	⬆️

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Impacted Files	Coverage Δ
mmrotate/apis/train.py	0.00% <0.00%> (ø)
mmrotate/core/evaluation/eval_map.py	53.10% <0.00%> (+45.51%)	⬆️
...otate/models/dense_heads/rotated_reppoints_head.py	6.82% <0.00%> (ø)
mmrotate/models/dense_heads/rotated_retina_head.py	16.66% <ø> (ø)
mmrotate/models/roi_heads/gv_ratio_roi_head.py	17.30% <0.00%> (+0.64%)	⬆️
...oi_extractors/rotate_single_level_roi_extractor.py	23.07% <0.00%> (-0.37%)	⬇️
mmrotate/models/roi_heads/roi_trans_roi_head.py	11.88% <0.00%> (-0.26%)	⬇️
mmrotate/models/dense_heads/rotated_atss_head.py	12.65% <12.65%> (ø)
mmrotate/models/detectors/base.py	30.30% <14.28%> (+5.30%)	⬆️
...ate/models/roi_heads/oriented_standard_roi_head.py	16.66% <14.28%> (-0.32%)	⬇️
... and 25 more

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