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) <[email protected]>

1 files changed, 8 insertions, 0 deletions

diff --git a/docs/_templates/sidebar/sponsor.html b/docs/_templates/sidebar/sponsor.html
new file mode 100644
index 000000000..db066a76c
--- /dev/null
+++ b/docs/_templates/sidebar/sponsor.html
@@ -0,0 +1,8 @@
+{# GitHub Sponsors button, shown just below the logo in the sidebar. #}
+<div class="sidebar-sponsor" style="display: flex; justify-content: center; padding: 0.4rem var(--sidebar-item-spacing-horizontal) 0.6rem;">
+  <iframe src="https://github.com/sponsors/hathach/button"
+          title="Sponsor TinyUSB"
+          height="32" width="114"
+          style="border: 0; border-radius: 6px;"
+          loading="lazy"></iframe>
+</div>