[Docs] Replace markdownlint with mdformat for avoiding installing ruby.

[ice-tong]

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Motivation

Replace markdownlint with mdformat to avoid installing ruby.
Refer to open-mmlab/mmcv#1936

Modification

*.md

BC-breaking (Optional)

No

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 MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.