Resolve "Think about where to put the archived versions"

With this MR, the semantic-release gitlab plugin is updated in order to allow for URLs as assets.

A link to the release files given at content/page is given in the release notes for easy access.

Furthermore, the old archiving mechanism has been removed in favor of providing a direct link to content/page in the navigation bar of the website. I also removed the versions and links before this one and added a link for the versions >= 1.0.0 to the navigation because a) the nav got a bit cluttered and b) versions lower than 1.0.0 haven't been considered as stable anyway and are not implemented anywhere.

All changes have been tested in a separate private repository.

Closes #56 (closed)