docs: Rename BLE to Bluetooth (LE) where applicable

The BLE acronym is not an official description of Bluetooth
LE, and the Bluetooth SIG only ever refers to it as Bluetooth
Low Energy or Bluetooth LE, so Zephyr should as well.

This commit does not change any board or vendor specific
documentation, and the term BLE may still be used in those.
It will be up to the vendors to update it if they want,
since many of them are using the term BLE in their
products.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>