Some improvements to CI Pipeline

This MR implements a couple of improvements for the CI pipeline:

• Split build and test (htmlproofer) into two stages
• Introduce scheduled job for external link checking
• Enable caching of Ruby gems
• Fix links that were not resolvable
• General speedup of the whole CI pipeline
• Introduce test group in .Gemfile to make html-proofer an optional dependency
• Reduce repetitions in gitlab-ci.yml
• Make html-proofer work without having to run jekyll serve

Closes #56 (closed)