Try to streamline 'quick start' documentation.

This should document really almost everything the user needs to do to get started with kdesrc-build.

The big thing missing is the .setup-env parts. I think that should go in kdesrc-build-setup but that's a separate discussion.

Also missing is discussion of --initial-setup, but then this is meant to be the push that helps define what all goes into --initial-setup. Once the documentation is accurate that should help in determining which features to include in kdesrc-build to be able to make this documentation even simpler, so that we can broaden the base of users able to be productive with this and related tools.