improve CI process for better testing

The current CI pipeline is on a single runner and should be added to VTK CI fleet (as it involves VTK master branch)

To be able to do that, here are the needed improvements:

• migrate to the structure of job definition from VTK
• work with VTK's CI to investigate suitable pinning strategies here (maybe bump weekly for some level of temporal stability (i.e., building today's commit in CI next year)? bump as needed with suitable commit messages?)
• prefer sccache to job caches (job caches are per-machine and disabled on the "standard" CI machines)
• use Kitware's DockerHub for image storage
• work against an installed VTK, not a build tree
• move job "build-gh-pages" to ensure it's done after deploying the examples