Better Setup and Security warnings UX

[barrelful]

Is your feature request related to a problem? Please describe.

After NC29, loopback access to the NC server instance is required for Setup and Security warnings. In the absense of this, many warnings are shown that could not check ... and also the upgrade documentation lacks more details on how to get it properly working.

Describe the solution you'd like
Based on the discussion on the forum, two improvements can be done:

1. there could be a pre-check if the domain is reachable instead of creating many could not check errors and make just one error to fix the connectivity first (or having a fallback, see alternative solutions).
2. he documentation Upgrade to Nextcloud 28 — Nextcloud latest Administration Manual latest documentation could have more details on the point

verifying that the Server can reach its own URL(s)

This could be an issue with the container/VM network or internal firewall or even some external firewall denying or dropping the request.

Describe alternatives you've considered
Any way to make the experience less frustrating and confusing to the server admin. Another way could be a fallback to previous checks without loopback connectivity like it worked previous to NC29 with a warning that the checks are not 100% reliable.

Additional context
More details on the discussion on the forum.