docs: add a Stages SB20 section to 22_devices_detail.md

[cauldnz]

The SB20 is a common bike with no entry in the device docs. Everything here is
from measurement on a real SB20 or from regular day-to-day use.

• Driver: advertises Stages Bike <NNNN>, handled by ftmsbike. No display of
  its own, so setup happens in the Stages app.
• Power/cadence: standard FTMS Indoor Bike Data (0x2AD2), flags 0x00c5. Worth
  noting bit 0 of those flags means instantaneous speed is NOT present, so the
  speed QZ shows is derived rather than reported by the bike.
• Auto resistance: fully supported over FTMS and reliable in regular use,
  including QZ-managed Peloton Power Zone workouts. Documented with the actual
  control-point exchange and its success responses.
• Rare issue: the bike can end up publishing a stripped Indoor Bike Data frame
  (flags 0x0011, distance only) with CSC stopped, so QZ correctly shows 0 W /
  0 rpm because that is genuinely all the bike is sending. A power-cycle
  restores it. Included because the symptom reads as a QZ bug when it is not.
  The trigger is NOT confirmed — a plausible cause is another consumer holding
  an FTMS control connection (a head unit or watch paired as a trainer rather
  than as sensors) — so it is written as a possibility, not a diagnosis.

Also relevant to the Equipment Compatibility wiki page, whose Stages row
currently reads Stages | ? | Yes | Yes | Yes | Yes | ? | No. Two corrections
worth making there: the model is the SB20, and Auto Resistance is Yes,
not No — ERG works and is relied on daily. HRM is still genuinely unknown to me,
so I would leave that as ? rather than guess.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com

[cagnulein]

Thanks!