Revise installation instructions for Fedora Atomic

[jedace07]

Updated installation instructions for Fedora Atomic derivatives, moving Aurora to be inline with Bluefin as it hasn't shipped Terra for over six months. Also slightly tweaked the install script, removing sudo from the layering command as it's not necessary, and replacing pkexec with run0 to accommodate for SecureBlue (or any other image that uses run0)

[GildedRoach]

Only SecureBlue uses run0 and every other Fedora derivative uses sudo, it would be more appropriate to add a SecureBlue section instead.

[jedace07]

run0 is available on all Fedora versions shipping systemd v256+ though, so imo it's unnecessary to create a dedicated script or section just for SecureBlue if the script works on any other Fedora atomic image

[GildedRoach]

They ship it because it's part of systemd, that does not mean that is what it's configured for or what is expected is in use.

run0 is not commonly known either, this is going to confuse less advanced users which is the opposite of what we want.

[nothingneko]

system policy outside of secureblue is not configured for run0, sudo is also still the recommended method across all of those listed variants

[jedace07]

Reverted the script back to pkexec and added a SecureBlue section