Board documentation in Doxygen

Dear RIOT maintainers,

Since #8516 [1] got finally merged, now we have the board documentation in Doxygen [2]. The information will still be available in the wiki for a while, but will be removed in further steps. I wrote some deprecation messages pointing to [2] so users can follow the new documentation.

For every new board there should be a doc.txt file with @defgroup, @ingroup and @brief directives and the board documentation in Markdown format (e.g see [3]).

Best regards,