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, 13 insertions, 0 deletions

diff --git a/docs/conf.py b/docs/conf.py
index c6d04bff5..7f670f082 100755
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -43,6 +43,19 @@ html_favicon = 'assets/logo.svg'
 html_theme_options = {
     'sidebar_hide_name': True,
 }
+# Furo default sidebar with a Sponsor button inserted just below the logo.
+html_sidebars = {
+    '**': [
+        'sidebar/brand.html',
+        'sidebar/sponsor.html',
+        'sidebar/search.html',
+        'sidebar/scroll-start.html',
+        'sidebar/navigation.html',
+        'sidebar/ethical-ads.html',
+        'sidebar/scroll-end.html',
+        'sidebar/variant-selector.html',
+    ]
+}
 html_static_path = ['_static']
 html_css_files = ['custom.css']