First things first: the recommend way is described in the wiki, which is also linked from our Website.
Github is mostly for people who need an installation for developing EGroupware.
That is a rather complicated method and only recommended for non Linux hosts, or exotic Linux distributions, for which we build no packages.
That’s the correct one, thought Fedora is not one of the distros we support / constantly test, but the RHEL repo could work.
No, it’s outdated, need to fix it. The tag is always the version, so for 21.1 it’s 21.1.