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

[MeowZheng]

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Motivation

• Fix mdformat version to support python3.6 and refine documentation.
• Remove ruby from CI

ref: open-mmlab/mmcv#2014

Modification

1. .github/workflows/lint.yml
2. .pre-commit-config.yaml

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1672 (1ed0ae3) into master (46326f6) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1672      +/-   ##
==========================================
- Coverage   90.25%   90.24%   -0.02%     
==========================================
  Files         142      142              
  Lines        8477     8486       +9     
  Branches     1428     1432       +4     
==========================================
+ Hits         7651     7658       +7     
- Misses        586      588       +2     
  Partials      240      240              

Flag	Coverage Δ
unittests	90.24% <ø> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
mmseg/apis/inference.py	61.66% <0.00%> (-2.13%)	⬇️
mmseg/models/backbones/vit.py	90.85% <0.00%> (-0.06%)	⬇️
mmseg/datasets/pipelines/test_time_aug.py	96.55% <0.00%> (+0.25%)	⬆️
mmseg/models/segmentors/encoder_decoder.py	88.66% <0.00%> (+0.31%)	⬆️

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