doc: extensions: api_overview: refactor extension

The extension had a some major design flaws, mainly:

- Ignored the `api_overview_doxygen_xml_dir` setting, instead it
  "sniffed" doxyrunner properties, so violating environment boundaries
- Computation of Doxygen HTML path worked because of the hardcoded URL
  in relative form found in doc/conf.py

This patch moves most code to the actual directive, so that context can
be obtained from the document being parsed. Also, the only config
required now is the Doxygen output dir, obtained from doxyrunner at
conf.py level.

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