docs: prepare v0.12.1 release notes and changelog

[wbarnha]

Description

Release-prep docs for v0.12.1, a packaging-only patch. v0.12.0's wheels
published to PyPI, but its source distribution was rejected under PEP 625
(hyphenated faust-streaming-0.12.0.tar.gz vs the required
faust_streaming-0.12.0.tar.gz). The code fix is #712; this PR is the notes.

• CHANGELOG.md — new ## [v0.12.1] section above v0.12.0, with a
  v0.12.0...v0.12.1 compare link, documenting the sdist fix (ci: build sdist with python -m build for a PEP 625-compliant filename #712).
• RELEASE_NOTES_v0.12.1.md — standalone GitHub release notes noting there
  are no functional changes (wheels-only installers are unaffected).

On "versioning files" and the Pages docs

There are no version files to bump — versioning is setuptools_scm-driven
(the git tag is the version), confirmed by grep (no VERSION file, no
hardcoded package version). The GitHub Pages docs reflect 0.12.1 with no
hardcoding:

• the changelog page is docs/changelog.rst → .. mdinclude:: ../CHANGELOG.md,
  so it renders the new v0.12.1 entry automatically;
• docs/conf.py derives the docs version from faust.__version__
  (setuptools_scm / tag), so the version banner updates to 0.12 on the next
  Pages build and the full release shows 0.12.1.

Release order (for the maintainer)

1. Merge ci: build sdist with python -m build for a PEP 625-compliant filename #712 (the actual sdist/PEP 625 fix) — required for the sdist to
   publish.
2. Merge this PR.
3. Tag v0.12.1 on master and create the GitHub release — the publish job
   rebuilds wheels + a correctly-named sdist (the publish step already sets
   skip-existing: true, so any 0.12.0 overlap is skipped).
4. Paste RELEASE_NOTES_v0.12.1.md into the release.

🤖 Generated with Claude Code

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.15%. Comparing base (411e6dd) to head (5b6aff7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #713   +/-   ##
=======================================
  Coverage   94.15%   94.15%           
=======================================
  Files         104      104           
  Lines       11136    11136           
  Branches     1201     1201           
=======================================
  Hits        10485    10485           
  Misses        550      550           
  Partials      101      101           

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