doc: Update kdesrc-build Getting Started docs.

Tried to bring in more coverage of the features users are actually
expected to use while eliminating discussion of things that have faded
away, especially now that the migration to Git is substantially
complete.

Some of this is based on user feedback I received earlier, noting that
the Getting Started guide (even as bad as it was), was the best part of
the documentation for them but that there were important things missing.

The docs after this section still need a bunch of cleanup, I think.