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>

CODE_OF_CONDUCT.md

@@ -14,8 +14,8 @@ project space.
 
 If you experience or witness behaviour that violates this code, please
 report it to the maintainers privately via email (see the contact in the
-project README), or via a private GitHub message to a maintainer. Reports
-will be handled confidentially and in good faith.
+project README). Reports will be handled confidentially and in good
+faith.
 
 ## Scope