Help: Reorganise module index into sections
- Split up module list into separate sections for utility modules and find modules.
- Improve wording to explain how the different module types are expected to be called/used.
- Move deprecated modules to their own separate sections.
There is also a separate commit that improves the linking of the Qt manual in the FindQt module (grew out of changes related to earlier versions of this MR).
Topic-rename: module-index-restructure