Document missing APIs

Instead of unspecific TBDs in the manual let's have a list here. I have added in braces what I think is the last we discussed:

Low level hardware access

• gyroscope (iio-sensor-proxy?)
• camera (v4l)
• vibration motor
• kill switch state (rfkill)
• audio
• gps
• bluetooth (bluez)
• LEDs

Higher level APIs

Some of these are handle by g-s-d (e.g. brightness by g-s-d power) others like gps have higher level APIs in GNOME as well. It's probably enough to point to upstream docs.

• send SMS, message/SMS retrieval (via chatty DBus)
• contacts retrieval (libfolks)
• initial call, phone list retrieval (via calls DBus)
• touch gestures
• ...