Rethinking site structure

I think we should rethink the site structure in https://develop.kde.org/docs

In particular the distinction of "Use", "Extend", and "Build" is more confusing than helpful IMO. I never quite know where to look for something.

The intention seems to be to separate between "Hacking on KDE stuff" and "Making third-party addons", but often that distiction isn't really possible. For example the KCM tutorial is both relevant for people creating or hacking on first-party KCMs as well as people looking to create a third-party one. Likewise the frameworks tutorials apply to both target groups.

We can try to come up with a better grouping of things, or do away with such grouping and have a more flattened list of items