Part of adopting a uniform naming policy for bridge variables,
where the variable prefix matches the role directory name.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Add Matrix <-> Meshtastic bridge (meshtastic-matrix-relay)

Vendors the meshtastic-matrix-relay (mmrelay) role into roles/custom/
following the conventions used by other bridge roles.

Co-authored-by: luschmar <90399580+luschmar@users.noreply.github.com>

* Add docs and CHANGELOG entry for Matrix <-> Meshtastic bridge

Co-authored-by: luschmar <90399580+luschmar@users.noreply.github.com>

---------

Co-authored-by: Slavi Pantaleev <slavi@devture.com>