Simplify ci

I think the pipeline rules in this repository might be a bit bloated. Or maybe not, so I'd welcome any feedback from @mguth, @pgadow, @svanstro, or anyone else. The main things I'm doing here:

• Run the same things regardless of context. I really don't like pipelines that only run on pushes upstream, because then the only way to test them is by pushing upstream.
• Remove unnecessary test stages (I wasn't sure why there is a docs_test and pages step.
• Remove a few unneeded packages we're installing for mkdocs.
• Use needs: for everything. This seems to speed things up a bit by executing everything in parallel.
• Decide what to do with the image creation. I don't currently see the need for them, so if we can run less code and put less stuff into registries we might as well. But maybe someone else is using them and can explain why we need them.
  • If we get rid of them, adjust the documentation to explain how to use the standard atlas images instead
  • If we keep them, add them back to this MR

OK, I'll wait for someone to explain why everything or something here is a terrible idea!