docs: Bluetooth: Move documentating for shell out of API

The API directory really isn't the best place for shell
documentation, which isn't API.

Created a new directory for Bluetooth shell documentation
and moved the ISO and LE Audio docs in there.

The existing bluetooth-shell.rst file was also split into
several new files, so that it follows the same structure.

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