Document custom alarm sounds

[bjorkert]

Adds an Alarm Sounds section under Alarms in the features guide, covering the tone picker and custom sound import.

• How to choose a built-in tone via Choose Tone.
• Importing custom sounds (Files picker or the app's shared folder), with the supported formats and the 2 MB / 30 s limits.
• Custom sounds are on-device only, excluded from settings export, and deletable, with fallback to a built-in.