Request: Various updates for installer pages

A few requests from @sxa as per the recent [July 2025 retrospective](https://github.com/adoptium/temurin/issues/84#issuecomment-3096337106):

Some "end user" installer notes:
- Linux install has the lines a little fragmented - perhaps we could have one box with all the commands that can be easily cut & pasted from?
- The initial apt commands to set up your machine before installing Temurin don't include `gpg` which is required and not present by default on the latest Ubuntu LTS
- We should have a note about STS vs LTS versions and `update-alternatives` ([discussion](https://adoptium.slack.com/archives/C9C272Z1T/p1752878253271079)) ref [mailing list post](https://accounts.eclipse.org/mailing-list/temurin-dev). At the moment 24 (STS) will take priority over 21 (LTS) if both are installed.

Also while this is in my head:
- The main installer page tells you `apt install temurin-21-jdk` or similar but is it obvious to the end user that there are other instructions (later on) to set up the repos?
- We should maybe make reference to the new Fedora/EL10 temurin repository package in the docs.