Resolve "Upload documentation to gitlab pages"

From now on, the ci/cd pipeline has to visible jobs (pages and build_pages) for building the documentation in build stage. Pages is executed on every commit, that has semantic versioning tag; otherwise build_pages is executed. build_pages just builds the documentation and doesn't generate any artifacts. pages on the other hand moves the generated documentation into an artifact. Gitlab then automatically runs a job called pages:deploy (in Deploy stage) that publishes the documentation to gitlab. Documentation will then be available at: https://kwh40.pages.rwth-aachen.de/fml40-reference-implementation/

NOTE: docs/source/requirements.txt got deleted. Haven't found anything using this besides gitlab-ci.yml. Neccessary package installation is now done there directly.

Closes #13 (closed)