doc: Add Google Programmable Search Engine

Add Google Programmable Search engine to the documentation, while
leaving it possible to easily revert to the built-in Sphinx search
engine. As the styling of the search results is apparently not thought
to be easy to tweak, the gcs.css stylesheet might require further
improvements.
Fixes #55173.

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