So that we can build Doxygen for docsets that are Sphinx+Doxygen.

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

- Update files to 1.9.6
- Use doxygen-awesome theme
- Use the same settings for all files, providing a uniform output

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

So that Doxygen part can be built standalone.

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

Now that we have Doxygen docsets, it is important to centralize
versioning into a single place, also re-using information from version
files or variables if available to avoid duplicates.

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