docs: Merge xsd-fu prerequisites into build instructions

Merge source code page with build instructions
Move build instructions to the top since it's got a wider
scope than contributing--non-developers need this
We can use a crossref to the top-level section, but in general we should not be using (internal to the docs) links to pages or files for this purpose--it's not a proper crossreference and isn't as useful in the PDF which benefits from proper crossreferencing.