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>

CONTRIBUTING.md

@@ -74,10 +74,10 @@ small worked examples.
 ## Testing
 
 There is no automated test harness for the firmware itself — testing is done
-by flashing onto real hardware and exercising the `wii5_buoy` and
-`wii5_modes` sketches. The `test/` directory contains smaller sketches
-demonstrating individual subsystems; please add a sketch there if you add a
-new driver.
+by flashing onto real hardware and exercising the `wii5_buoy` sketch. The
+`test/` directory contains smaller per-subsystem sketches (Iridium, Sparton,
+GPS, DS18B20, BinData, console commands, pin diagnostics); please add a
+sketch there if you add a new driver.
 
 In your PR, describe:
 - What you tested on (hardware or just compile?)