bluetooth: audio: shell: Improve indentation when printing codecs

Codecs can be printed at different levels:
level 0: Generic and unicast
level 1: subgroup codec configs
level 2: BIS codec configs

This commit ensures that the indentation level
is correctly increase based on where the codec configuration
data is printed.

This also updates the documentation for the BAP shell.

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