[Docs] Replace markdownlint with mdformat for avoiding installing ruby

[LeoXing1996]

Refers to open-mmlab/mmcv#1936

[codecov]

Codecov Report

Merging #298 (1a15109) into master (10cd8f1) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #298   +/-   ##
=======================================
  Coverage   74.78%   74.78%           
=======================================
  Files         145      145           
  Lines       10912    10912           
  Branches     2145     2145           
=======================================
  Hits         8160     8160           
  Misses       2214     2214           
  Partials      538      538           

Flag	Coverage Δ
unittests	74.78% <100.00%> (ø)

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Impacted Files	Coverage Δ
mmgen/__init__.py	70.58% <100.00%> (ø)

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[plyfager]

The version of MMCV and MMCLS changed. mdformat did these?

[LeoXing1996]

The version of MMCV and MMCLS changed. mdformat did these?

I did this manually. Otherwise, all unit tests will be failed because in __init__.py we check the mmcv version.