boards: silabs: Create subcategories for different board types

Introduce three subcategories for Silabs boards: dev_kits, radio_boards
and starter_kits. The dev_kits category contains small form-factor
standalone boards branded as "Dev Kit" or "Thunderboard". The
radio_boards category contains radio boards to be used with a wireless
mainboard, and the starter_kits category contains larger form-factor
standalone boards branded as "Starter Kit".

Structure the Sphinx documentation the same way as the directory tree.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>