doc: rename iot-lab_M3 group name
doc: add names to pca1000x boards
doc: improve doxygen cache size
doc: beautify HTML documentation

* add logo
* add description
* use RIOT specific color scheme
* add search functionality
* remove broken custom header, footer, and style sheet
SQUASH ME: change from green to red
doc: remove more CPU specific documentation
FIX UP for broken thread_init