docs: add Practice mode user guide

[pliski]

Companion to the Practice-mode proposal (discussion: beatboxjs/ror-player#150;
code: beatboxjs/ror-player#151). Draft, for discussion — see the issue for context.

What this adds

A user-guide page for the new Practice mode (companion to the app PR
beatboxjs/ror-player#151).

The page (src/guide/user/practice.md) walks through:

• getting started (mic + headphones, picking a tune / part / instrument),
• the Instrument vs Band practice modes,
• reading the live feedback (score, timing meter, session totals),
• the notation key — green / amber / red strike-through (miss) / dotted outline (extra),
• the settings (speed, latency calibration, sensitivity, difficulty),
• and how the score is understood (qualitatively; latency vs drift).

It also wires the page into the sidebar (config.ts), the introduction index
(README.md), and registers the microphone icon (client.ts).

yarn build passes with no dead links.