docs: convert ReST (Sphinx) to Markdown (MkDocs)

[andreoliwa]

Proposed changes

1. Convert all docs from ReST to Markdown

Checklist

• Read the contribution guidelines
• Run make locally before pushing commits
• Add tests for the relevant parts:
  • API
  • CLI
  • flake8 plugin (normal mode)
  • flake8 plugin (offline mode)
• Write documentation when there's a new API or functionality

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.16%. Comparing base (a98c439) to head (c25940a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #725   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files          24       24           
  Lines        2166     2166           
  Branches      275      275           
=======================================
  Hits         2083     2083           
  Misses         55       55           
  Partials       28       28           

Flag	Coverage Δ
macos-latest	96.02% <100.00%> (ø)
ubuntu-latest	96.02% <100.00%> (ø)
windows-latest	96.02% <100.00%> (ø)

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[github-actions]

🎉 This PR is included in version 0.38.1 🎉

The release is available on:

• v0.38.1
• GitHub release

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