Move repo links from gitlab to github

[VanillaBrooks]

There are still a number of links pointing towards the gitlab repository mirror instead of the upstream github. Here is one example:

acts-project.github.io/mkdocs.yml

Lines 15 to 20 in b0b395b

	url_imports:
	license: https://gitlab.cern.ch/acts/acts-core/raw/master/LICENSE
	readme: https://gitlab.cern.ch/acts/acts-core/raw/master/README.md
	getting_started: https://gitlab.cern.ch/acts/acts-core/raw/master/getting_started.md
	contributing: https://gitlab.cern.ch/acts/acts-core/raw/master/CONTRIBUTING.md
	authors: https://gitlab.cern.ch/acts/acts-core/raw/master/AUTHORS.md

This is the most notable issue (apologies for MS paint skills):

A quick grep shows 33 gitlab links. Should these going to be updated to the new upstream or are they intentionally kept on gitlab?