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>

Doxyfile

@@ -10,8 +10,8 @@
 # --- Project identity --------------------------------------------------------
 PROJECT_NAME           = "WII5 Buoy Firmware"
 PROJECT_BRIEF          = "Firmware for the WII5 wave-measurement buoy (ATmega2560)."
-# PROJECT_NUMBER is normally injected by the build script:
-#   doxygen <(sed "s/^PROJECT_NUMBER.*/PROJECT_NUMBER = $(cat VERSION | grep version | cut -d= -f2)/" Doxyfile)
+# PROJECT_NUMBER can be injected at invocation time, e.g.
+#   doxygen <(sed "s/^PROJECT_NUMBER.*/PROJECT_NUMBER = $(git describe --tags --always)/" Doxyfile)
 # Hardcoded fallback follows.
 PROJECT_NUMBER         = v5.5.x