[Mycat-04]

Add a guide for building and running upstream RetroArch on the ArmSoM CM5 development kit with its native Debian 12 XFCE ARM64 image, and add an entry under Advanced in the documentation navigation.

The guide includes the tested upstream revision, dependencies, EGL/OpenGL ES build options, a user-local installation, HDMI audio and keyboard configuration, and a reproducible homebrew game example. RetroArch itself is built from source; the Nestopia core used for the example is extracted from a Debian package.

Hardware validation was performed on October 22, 2026, with kernel 6.1.118-rk3576 #198 and RetroArch 1.22.2 at a81e450c00745b27ba417d486799b12f09c38ea4. Display, keyboard controls, HDMI audio, menu navigation, pause/resume, save/load states, and exit/relaunch were verified with Falling and Nestopia. No RetroArch source changes were required. The guide states the limits of this test rather than extending the result to other images or cores.

Documentation validation:

• Bash syntax checks passed for all seven command blocks.
• The new navigation entry resolves to the added guide.
• git diff --check passed.
• The complete documentation site built successfully with mkdocs build --strict (exit code 0).

Related platform listing: libretro/RetroArch#19900.