docs: document sound cues and the Volume setting (WP2)

[brickbots]

WP2 of the rev4 documentation pass (rev4-docs-update-plan.md §4).

Why

rev4 has a passive piezo buzzer, and Settings → User Pref → Volume (Off, 1–5) shipped
in v2.6.1 with zero documentation — it appears in neither user_guide.rst nor
menu_map.rst. A rev4 owner who heard a beep had nowhere to look, and no way to find the
off switch. This closes that gap.

What changed

docs/source/user_guide.rst — new Sounds sub-section at the end of Settings Menu:

• what the tones mark: each key press, startup, shutdown, and every low-battery warning;
• where Volume lives, its range, and that choosing a level plays a sample tone at that
  level so you can set it by ear;
• the honest caveat — the buzzer is far louder at some pitches than others, so cues differ
  in loudness by design, not as a fault;
• how to turn it off;
• a .. note:: scoping v3/v2.5 (no buzzer), per D1's rev4-as-default rule.

docs/source/menu_map.rst

• Volume added to the User Pref definition list, next to Key Bright, matching the
  on-device order in menu_structure.py.
• GPS Baud Rate now says which receiver each speed belongs to: rev4's 10th-generation
  UBlox at 115200, the older v3/v2.5 receiver at 9600 (added mid-task at Rich's request).

Screenshots — new volume_setting_docs.png at 176 px, plus the three existing
Settings Menu shots re-taken at 176 px per D3 rule 3. settings_03.png was also stale:
it showed PiFinder Type / Camera Type at the top level, from before they moved under
Advanced. All four are 352×352 after screenshot_to_doc.py --scale 2, captured with -fb
so the title bar carries the rev4 battery icon.

Vocabulary

The Sound glossary's term is earcon; that stays a domain term. User-facing prose says
"sounds" and "tones", and "earcon" appears nowhere in the manual.

One deviation from the plan, flagged for review

The plan (§4 WP2 task 2) lists errors among the cues and says the setting "only
appears on hardware that has the buzzer." Neither matches the code, so I documented what
ships:

• No error tone is wired. Earcon.ERROR and SOLVE_LOCK have catalog entries but
  python/PiFinder/sound.py marks them "defined-but-unwired in v1", and the only
  producers in main.py are STARTUP, SHUTDOWN, KEYPRESS and LOW_BATTERY (plus
  VOLUME_SAMPLE on a volume change). Documenting an error tone would have sent owners
  listening for a sound that never plays.
• The Volume menu item is not hardware-gated. It is unconditional in
  menu_structure.py; what is gated (main.py:555) is the sound process
  (capabilities.has_buzzer and hardware_platform == "Pi"). So on a v3 the setting is
  present and simply does nothing — which is what the note now says.

Verification

Full clean rebuild against an empty build dir, nitpicky mode, with the docs venv
(Sphinx 7.2.6 + sphinxcontrib-mermaid):

sphinx-build -b html -n docs/source /tmp/pifinder_docs_build_wp2

Zero warnings, zero errors. Baseline was already clean, and menu_map.rst's mermaid
diagrams all build.

Scope

docs/source/*.rst only. No docs/*.md stubs, no docs/ax/*, no ADRs, no
build_guide.rst / BOM.rst (D4). Deliberately kept small to merge cleanly alongside the
sibling WP1/WP3/WP4 branches.

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