ci(docs): replace markdown-cli in favor of pymarkdownlnt

[qlrd]

markdown-cli is low maintained and pymarkdownlnt appears to be a good choice given its features. Also, with this change, the CHANGELOG was changed.

[Copilot]

Pull request overview

This PR replaces the low-maintained markdown-cli tool with pymarkdownlnt for markdown linting in the CI pipeline. The change consolidates markdown linting into the main Poetry-based toolchain and updates the CHANGELOG with recent project changes.

• Adds pymarkdownlnt as a dev dependency with appropriate rule configuration
• Removes the separate GitHub Actions markdown linting job in favor of Poetry task integration
• Updates CHANGELOG.md with version 0.0.21 entries and improves formatting consistency

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
pyproject.toml	Adds pymarkdownlnt dependency and integrates markdown linting into the Poetry lint task sequence
.github/workflows/ci.yml	Removes the standalone markdown linting job (job2) that used markdownlint-cli
CHANGELOG.md	Adds version 0.0.21 entries and converts inconsistent bullet formatting to standard hyphenated lists

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.60%. Comparing base (3dd1c9a) to head (8a21c64).
⚠️ Report is 33 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #201      +/-   ##
==========================================
- Coverage   98.76%   98.60%   -0.16%     
==========================================
  Files          63       63              
  Lines        3167     3236      +69     
  Branches      273      284      +11     
==========================================
+ Hits         3128     3191      +63     
- Misses         24       26       +2     
- Partials       15       19       +4     

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