doc: boards: extensions: introduce zephyr:board role and directive

A new zephyr:board:: Sphinx directive allows  to flag a documentation
page as being the documentation for a specific board, allowing to
auto-populate some of the contents, ex. by adding a board overview a la
Wikipedia, and later things like supported HW features, etc.

A corresponding :zephyr:board: role allows to link to a board doc page.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>