Qt Wayland documentation overhaul

There have been some feedback that it's hard for beginners to get into Qt Wayland and Qt Wayland Compositor through the documentation. We have quite a lot of high quality documentation, but something seems to be missing to make it complete.

One thing is the introduction to Wayland itself. Our documentation pretty much starts with the question "do you want to make a multi-process platform for embedded, then you need Wayland", but never really explains the categories or terms used, and it never really introduces the technology.

In addition, there may be links missing from landing pages etc. to make the first impression better.

And finally, there is some class documentation that is not really complete with some typical recursive definitions etc.

This task is to go through everything and tie stuff together and clean up a bit, try to make the great docs we have more visible.