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, 4 insertions, 4 deletions

diff --git a/tools/make_release.py b/tools/make_release.py
index 71c1e6f64..d27ce20b3 100755
--- a/tools/make_release.py
+++ b/tools/make_release.py
@@ -3,7 +3,7 @@ import re
 import gen_doc
 import gen_presets
 
-version = '0.20.0'
+version = '0.21.0'
 
 print('version {}'.format(version))
 ver_id = version.split('.')
@@ -63,7 +63,7 @@ gen_doc.gen_boards_doc()
 # gen presets
 gen_presets.main()
 
-##################(ver#
-# docs/info/changelog.rst
 ###################
-print("Update docs/info/changelog.rst")
+# docs/info/changelog/
+###################
+print("Add docs/info/changelog/{}.rst and list it at the top of docs/info/changelog/index.rst".format(version))