New user-facing feature & usage guide covering inherited OpenGD77 features and
FreeTRX additions, each marked [FreeTRX] or [OpenGD77]. Documents the spectrum
scope (VFO sweep, hold #), channel add/delete, mute, screen dimming, the S-meter
and 7-seg font, plus a button reference. Linked from the README.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
