Feature: Run jekyll locally

[m-bock]

As discussed in #40, here's a draft that enables previewing the generated docs locally.

NOTE on formatting in the README.md: Excuse me for the unclean diff, but that's what prettier produces when applied with the local prettier configuration. So I think it would make sense to stick to that. But I can revert it, too.

I'll provide more detailed explanations in the comments soon. (edit: done)

[m-bock]

Did some updates, explanation will follow...

[m-bock]

Sorry guys, I have not forgotten about this PR. I got overwhelmed by other tasks in the meanwhile. But I'll come back to finish this one soon.