Bump version to 0.21.0; rework changelog, contributors, and docs

Release:
- Bump version to 0.21.0 and regenerate tusb_option.h, library.json,
  repository.yml, sonar-project.properties, boards.rst, dependencies.rst
- Add 0.21.0 release notes and split the changelog into per-release files
  under docs/info/changelog/ (date out of title, driver/class groups as
  sub-headings, DCD & HCD section after Device/Host stacks)

Contributors:
- Credit each release's PR authors in a Contributors section
- Drop the curated contributors page; add MAINTAINERS.rst

Docs:
- Update Code of Conduct to Contributor Covenant 3.0 (keep it in the repo,
  remove it from the built docs)
- Sidebar: add a home entry, rename the group to "Documentation", move
  Changelog into it, add a GitHub Sponsor button, merge external links,
  rename FAQ; hide the inline toctree on the landing page
- Add the make-release skill and update the AGENTS.md release process

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

MAINTAINERS.rst

@@ -0,0 +1,10 @@
+***********
+Maintainers
+***********
+
+- `Ha Thach <https://github.com/hathach>`__ — author and maintainer
+- `Zixun Li (HiFiPhile) <https://github.com/HiFiPhile>`__ — maintainer; audio expertise
+
+Everyone who has contributed is credited per release in the
+`changelog <docs/info/changelog/index.rst>`__ and in the GitHub
+`contributors graph <https://github.com/hathach/tinyusb/graphs/contributors>`__.