doc: _extensions: add board target selector for supported HW features

This show a nice widget to switch between the various board targets and
see their respective list of supported features.

The original HTML content of the page is preserved and JavaScript code
"patches" the page on-the-fly. This is so that the actual HTML content
(that e.g. search engines see) is complete and indexable (as well as to
provide useful information for folks who might have JavaScript disabled
altogether).

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