CI: Remove needless build stage, document a working full build.
The commented portion (when turned into an equivalent Dockerfile) works with latest kdesrc-build to get all the way through a build of dolphin. But it takes quite some time so probably not appropriate for CI to run.
For CI purpose probably build very simple application then dolphin? Or even simple tier1 or tier2 framework?
I'm thinking a simple framework or two. Maybe even a shell framework which exists just for testing the KDE cmake machinery. kdesrc-build itself could serve that function perhaps as it will build without any of the KF5 installed but will use KDocTools if present.