Replace xi:include with inherited member entities and resolve cross-references

[jordikroon]

See commits in order.

I honestly stretched this bit too much than I should, and it requires a PHD patch that I have ready locally. But before I push that through..

I want to get opinions on this before we get to the nit part. Ideally from @alfsb or @Girgias. Is this the most ideal method, or do we have alternatives? Imho the xi:include part is alright.

Another important (follow-up) would be to fix the types (int, string, etc). They exist within the class synopsis as well as standalone. What options do we have for that that don't feel so (w)hacky?

[alfsb]

What I probably would do in this case is have one or some "local" page for all jump links. That is, a local page for basic types, exceptions and other classes, so links clicked on extensions manual would not immediately jump between manuals. They first land in the local page, where there is a very small description of such type/class, and then a jump link in the language manual for further details.

So entity and <type> usage would remain the same on the files, and also the distinction of manual would be a little more visible,

A jump link, that immediately jumps to a page with identical visual CSS/rendering, has the potential of confusing a lot of people after an "invisible" manual jump.

So doc-extensions would have a language/ dir, containing things like language.types.xml, language.exceptions.xml and so on.