chore(doc): remove unmaintained docs/CHANGELOG

[danceratopz]

The docs/CHANGELOG.md file is a leftover from the ethereum/execution-spec-tests repository and has not been properly maintained since the merge into ethereum/execution-specs. Its contents do not match any of the new changelog formats proposed in issue #2036 and only receive sporadic updates, so the file in its current shape is more confusing than useful.

Remove the changelog along with all references to it: the just changelog recipe, its tox migration shim, the CI workflow job, the validate_changelog entry point and helper function, the Changelog project URL, the section template, and the docs developer instructions. The file history remains in git should it need to be restored.

🔗 Related Issues or PRs

Related to: #2036

✅ Checklist

• All: Ran fast static checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  just static

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.62%. Comparing base (5cdb055) to head (55c17d7).

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #2810   +/-   ##
================================================
  Coverage            88.62%   88.62%           
================================================
  Files                  577      577           
  Lines                35659    35659           
  Branches              3490     3490           
================================================
  Hits                 31604    31604           
  Misses                3492     3492           
  Partials               563      563           

Flag	Coverage Δ
unittests	88.62% <ø> (ø)

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