* 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>

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