docs: fix dangling references, document Gitea/GitHub mirror split

CONTRIBUTING.md: remove the deleted wii5_modes sketch from the testing
guidance; mention the surviving test/ subsystem sketches instead.

README.md: drop the now-incorrect "hardware schematics" entry from the
repository-layout block (doc/hardware/ was removed earlier). Add a
Repository section naming the Gitea canonical and the GitHub mirror
(https://github.com/SH3D/WII5Firmware) used for community issues and PRs.

SECURITY.md, CODE_OF_CONDUCT.md: point at the GitHub mirror for security
advisories; drop the vague "private GitHub message" path from the CoC.

CHANGELOG: replace the "TODO" placeholder with a real v5.5.1 initial
public release entry.

Doxyfile: rewrite the PROJECT_NUMBER injection example to use
`git describe` instead of the deleted VERSION file.

VERSION: removed. It was bumped by tools/tag_version.sh +
tools/build_version.sh (both deleted in c89c636); build_local.sh injects
WII5_SOFTWARE_VERSION from `git log -1 --pretty=%h` at compile time, so
nothing load-bearing depends on the file.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -112,7 +112,7 @@ A running log of notable real-world deployments lives in `DEPLOYMENTS.md`.
 app/wii5_buoy/        Arduino sketch entry point
 WII5*.{h,cpp}         Firmware source (board root, see Architecture)
 WII5_board_v2.h       Pin/peripheral map for the WII5 v2 board
-doc/                  Design notes, command reference, hardware schematics
+doc/                  Design notes, command reference, generated API docs
 test/                 Smaller per-subsystem test sketches
 tools/                Build, flash, and size-profile helper scripts
                       (env-var driven; copy to *.local.sh to set local
@@ -125,10 +125,24 @@ See [CONTRIBUTING.md](CONTRIBUTING.md). Please also read
 [CODE_OF_CONDUCT.md](CODE_OF_CONDUCT.md) and [SECURITY.md](SECURITY.md)
 before opening issues or PRs.
 
+## Repository
+
+The canonical source is hosted on Gitea:
+
+- **Main repository:** https://gitea.sh3d.com.au/Sh3d/WII5Firmware
+
+A read-only mirror is published on GitHub for community issues and pull
+requests:
+
+- **GitHub mirror:** https://github.com/SH3D/WII5Firmware
+
+Open issues and PRs against the GitHub mirror; changes are reviewed there
+and merged back through the Gitea canonical.
+
 ## Contact
 
 - Maintainer: Scott Penrose <scottp@dd.com.au>
-- Bugs and feature requests: GitHub Issues
+- Bugs and feature requests: [GitHub Issues](https://github.com/SH3D/WII5Firmware/issues)
 - Security issues: see [SECURITY.md](SECURITY.md)
 
 ## License