doc: extensions: add kconfig search extension

Add a new extension to handle Kconfig documentation. This means that no
more CMake hackery is required. However, the way it works differs from
the current approach. Instead of creating a single page for each Kconfig
option, the extension creates a JSON "database" which is then used on
the client side to render Kconfig options on a search page. The reason
to go for a single page choice is because Sphinx is significantly slow
when handling a lot of pages. Kconfig was responsible for an increase of
about ~10K pages.

Main features:

- Generates a Kconfig JSON database using kconfiglib APIs.
- Adds a new Sphinx domain for Kconfig. The domain provides a directive,
  :kconfig:search:: that can be used to insert a Kconfig search box onto
  any page. This page is where all Kconfig references inserted using the
  :kconfig:option: role will point to. The search functionality is
  implemented on the client side using Javascript. If the URL contains a
  hash with a Kconfig option (e.g. #CONFIG_SPI) it will load it.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>