[Docs] Replace markdownlint with mdformat for avoiding installing ruby

[MeowZheng]

related pr open-mmlab/mmcv#1936

[codecov]

Codecov Report

Merging #130 (7b0afe0) into master (a014976) will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
- Coverage   69.09%   68.95%   -0.14%     
==========================================
  Files          92       92              
  Lines        5135     5135              
  Branches      727      727              
==========================================
- Hits         3548     3541       -7     
- Misses       1473     1479       +6     
- Partials      114      115       +1     

Flag	Coverage Δ
unittests	68.95% <ø> (-0.12%)	⬇️

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

Impacted Files	Coverage Δ
mmflow/utils/misc.py	95.23% <0.00%> (-4.77%)	⬇️
mmflow/datasets/pipelines/transforms.py	79.16% <0.00%> (-1.00%)	⬇️
mmflow/datasets/dataset_wrappers.py	43.75% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f3d9449...7b0afe0. Read the comment docs.