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>

SECURITY.md

@@ -9,9 +9,10 @@ control of a deployed buoy — **please do not open a public GitHub issue**.
 
 Instead, report it privately:
 
-- Open a private security advisory via the GitHub repository's
-  **Security** tab → "Report a vulnerability", or
-- Email the maintainer: Scott Penrose <scottp@dd.com.au>
+- Open a private security advisory via the GitHub mirror's **Security**
+  tab → "Report a vulnerability":
+  https://github.com/SH3D/WII5Firmware/security/advisories/new
+- Or email the maintainer: Scott Penrose <scottp@dd.com.au>
 
 Please include: