documentation: CMSIS cleanup

Put all CMSIS documentation into a dedicated subgroup of cpu.
documentation: moved network documentation to top level
documentation: added missing doxygen header to transceiver interface
documentation: fixed organization of 6LoWPAN doxygen
documentation: fixed sixlowpan doxygen filename
Merge pull request #1015 from OlegHahm/sixlowpan_doxygen_fix

Some doxygen fixes