docs: cut the SnapShots guides to what a reader needs

The user guide narrated UI states, env vars, D-Bus versions, and tool
binaries. It now explains how to turn the feature on, capture, change
the shortcut, and what each Linux desktop needs. The internals page
kept only cross-component traps a maintainer would otherwise rediscover.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: juliusmarminge