doc: extensions: boards: Better handle unknown/other vendors

Boards under a folder that doesn't directly match a vendor prefix
incorrectly end up being categorized as "zephyr" since that's the only
prefix that is eventually found when navigating up the folder hierarchy.

This commits treats boards in the "others" and "native" folders as
special cases and associates them to an "Unknown/Other" category.

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