Place "Concepts" between "General" and "For Users" in the sidebar

[ma-hartma]

Description

As the concepts part explains metal-stack, the birds-eye-view and all the lingo, it makes sense for it to come before the deployment guide and all the specific docs.

I would have already moved it in #310, but there are to many PRs open and I don't want to start a rebase-party after renaming all directories.
Waiting for a better moment.