doc: extensions: kconfig: improve search capabilities

This patch modifies the Kconfig search _algorithm_ so that it aligns
with how menuconfig search works:

- Input is splitted into multiple search terms (space based split)
- Both Kconfig option name and prompt are used
- Regex search is used instead of match, it is less efficient but will
  search the whole string, leading to more results.

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