Add firmware artifacts, memory summary, and Doxygen gh-pages deploy

CMakeLists.txt: generate firmware.map via -Wl,-Map.

CI build job:
- Writes per-section size table to the Gitea job summary
  (visible in the Actions UI after each run).
- Uploads firmware.hex and firmware.map as a downloadable artifact
  named firmware-<sha>.

CI docs job:
- On push to main, force-pushes docs/html/ as an orphan commit to
  the gh-pages branch.  Gitea Pages must be enabled in site admin
  for the HTML to be served; the branch is always available via the
  repo file browser regardless.

CMakeLists.txt

@@ -56,6 +56,7 @@ target_link_options(firmware PRIVATE
     -T ${CMAKE_SOURCE_DIR}/link/nrf52840.ld
     -Wl,--gc-sections
     -Wl,--print-memory-usage
+    -Wl,-Map=${CMAKE_BINARY_DIR}/firmware.map
     -nostartfiles
     -specs=nano.specs
     -specs=nosys.specs