bluetooth: mesh: fix some Doxygen groups location

Bluetooth mesh had many groups defined as top level Doxygen groups.
Place all of them to `bt_mesh`. Also removed redundant `@brief` entries,
and placed the block at the right location (just below extern "C"
entry), as well as uniform spelling of "Mesh".

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>