Adapt to gitlab.com migration

This MR adapts the project structure now that dantro is hosted on gitlab.com, see #296 (closed)

Can this MR be accepted?

• Remove default tags
• Remove custom docs deployment
• Adapt docs and README
• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up Squash option set
  • Changelog entry added (not necessary)
  • Version number bumped (not necessary)