[Docs] Fix mdformat version to support python3.6 and remove ruby install

[MeowZheng]

ref open-mmlab/mmcv#2014

[codecov]

Codecov Report

Merging #154 (b916143) into master (8aee672) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   68.82%   68.86%   +0.03%     
==========================================
  Files          92       92              
  Lines        5148     5148              
  Branches      733      733              
==========================================
+ Hits         3543     3545       +2     
+ Misses       1492     1488       -4     
- Partials      113      115       +2     

Flag	Coverage Δ
unittests	68.86% <ø> (+0.09%)	⬆️

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Impacted Files	Coverage Δ
mmflow/datasets/dataset_wrappers.py	43.75% <0.00%> (ø)
mmflow/datasets/pipelines/transforms.py	79.83% <0.00%> (+0.33%)	⬆️

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