Switch to GH actions to build GH pages

[ascheman]

Does this help to solve issue #15, @dasniko?

I had to switch also to Jekyll 4 as 3.8 ran into weird dependency problems on the GH action runner.
Before merging, I would squash / re-comment some of the commits ...

[dasniko]

As far as I can see, this would completely customize the build process, no more automated GitHub process, no more convenience. If something doesn't work, we have to fix it on ourselves.
I really like the approach of easiness and making benefit of using a platform and not having to do everything on our own.

[dasniko]

There are so many questions to me, when I look at your commits:

• Why a custom Dockerfile as it jus doesn't do anything different from the public one? Or do I oversee something?
• Why is the Gemfile.lock checked in? Isn't that something you don't want to checkin? At least, I learned it that way.
• Why did you remove the volume from the docker-compose.yml? This saves a lot of time for local development, having a persistent volume with pre-loaded libs.