Optimize HTMLProofer options

Currently, we check, if external links are properly resolved and also expect to have the .html extension, which we do not require. Jekyll, as well as our future production setup, as well as GitLab Pages don't require that. Furthermore, dead links are somehow not properly checked. There are a plenty of dead links, e.g. on the frontpage at the moment, but the pipeline succeeds.

I suggest to run the htmlproofer with the following options. Like this, we also don't need to have a local web server running.

$ bundle exec htmlproofer \
    --internal-domains localhost:4000 \
    --assume-extension \
    public

We could also discuss, if we should check for external links in a seperate pipeline, that is only running once a week for example. Otherwise our jobs could fail, only if a dependent service is currently offline.

I will submit a MR in a minute, to see if my assumptions are correct.