Investigate the feasibility of introducing rst/sphinx to generate user documentation in addition to DocBook.

It would require a different code path for PDF and cache generation, or direct support in khelpcenter. If the cache can be generated, maybe no other changes are needed.

That will require a custom sphinx extension:

• to use custom common "entities" that should be extracted separately
• (maybe) to generate the cache, if it is possible to create a compatible one

It will also require support for KDocTools for common entities.