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>

CHANGELOG

@@ -1,30 +1,12 @@
-v5.x - 2024
-* TODO
+# Changelog
 
-v1.x
-* New BinData for sending/receiving binary data on the Iridium
-* Simplified and improved the button logic.
-* New logging level controls
-* GPS Debug and Passthrough in any mode
-* Better beep for Maths Start
-* Rugged Position Mode
-* Calculate time until next...
+## v5.5.1 — 2024
 
-v1.0.20
-* Battery And Temperature encouraged to read near boot timed
-* Hold/Click button cycle Improved
-* Power for Buzzer
-* Hold feedback messages for Displaying shutdown time of Maths
-* Sleep period configurable in EEPROM
-* Split non-essential loop out for Maths
-* Mode timeouts
-* Direct maths Hold command
-* Fully working Seep Mode
-
-v1.0.16
-* statusDump - automatically shown at Voltage and Weather to allow Local Server updates
-* Using @WII5,setting,mode,default,{STRING} has worked for a while, added "swap" which switches
-   to sleep if it was capture, otherwise to capture.
-* Updated Test help to remove incorrect documentation.
-* Large number of experimental changes in Maths to facilitate start/stop/reboot
+Initial public open-source release under the Apache License 2.0.
 
+- Firmware merged from the legacy WII3 (watchdog / maths-CPU) and WII1
+  (buoy I/O) lineages.
+- Target: ATmega2560 on the custom WII5 v2 carrier board.
+- Modes: Capture, Position, Sleep, ManualTest, SelfTest, LowBattery.
+- Iridium 9602 / 9603 SBD telemetry; Sparton AHRS-M1/M2 IMU; u-blox
+  NEO-6M/7M GPS; dual SD storage.