docs: prepare v0.12.0 release notes, changelog, and docs version

[wbarnha]

Description

Release-prep documentation for v0.12.0. No product code changes —
versioning is setuptools_scm-driven, so the git tag is the version; this PR
prepares the human-facing notes and makes the docs report the right version.

Scope: merged-only. The changelog/release notes describe only what is
already on master. Not-yet-merged work — the offset-commit data-loss fixes
(#606/#707, #316/#692), the optional OpenTracing/OpenTelemetry extras
(#685/#686, #688), web_application_options (#704), and the reported-issue fix
stack (#693–#703, #705) — has been removed and will be folded in as it merges.

• CHANGELOG.md — resumes the Keep a Changelog format (dormant since
  v0.8.10) with a ## [v0.12.0] section (Added / Changed / Fixed /
  Dependencies) and a v0.11.3...v0.12.0 compare link. Date left as
  UNRELEASED; set it at tag time.
• RELEASE_NOTES_v0.12.0.md — standalone GitHub release notes with
  highlights and a Breaking changes / upgrade notes section.
• docs/conf.py — Sphinx docs version now derived from faust.__version__
  (setuptools_scm / git tag) instead of the hardcoded robinhood-era
  1.1/1.0, so the published Pages docs report the real version.

Merged highlights in v0.12.0

• confluent-kafka driver re-added (confluent://, faust[ckafka]).
• Python 3.13 and 3.14 supported; 3.8 and 3.9 dropped (EOL support for Python 3.8 #650).
• aredis replaced by redis-py (Replace aredis with redis-py #635).
• Live-broker Kafka harness in CI (ci: add live-broker integration test harness (Kafka in CI) #706); modernized CI/wheels (Modernize CI: wheel runners, Windows wheels, and test matrix (drop 3.9, require 3.14) #690).

Dependencies noted

• mode-streaming >= 0.4.0, aiokafka >= 0.10.0 (compatible with recent
  0.13/0.14 — fix: support aiokafka 0.13.0 which removed api_version parameter #674, fix(aiokafka): keep Faust assignor for changelog tables; drop Python 3.9 #682), new confluent-kafka >= 2.0.0 for faust[ckafka],
  and the faust-cchardet fork replacing unmaintained cchardet.

Tagging checklist (for the maintainer)

1. Merge this PR.
2. Set the UNRELEASED date in CHANGELOG.md to the tag date.
3. Tag v0.12.0 on master and push — setuptools_scm picks up the version, the
   wheel/publish job runs, and the rebuilt Pages docs report 0.12.
4. Paste RELEASE_NOTES_v0.12.0.md into the GitHub Release.
5. As the pending fix stack merges, append those entries to the changelog.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.15%. Comparing base (ade8a09) to head (645d942).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #708   +/-   ##
=======================================
  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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