user invites: Tell site admins when emails are not configured

Created by: philipp-spiess

Closes #32308 (closed) Depends on #32425

@rrhyne @slimsag Opening this PR so we can have a quick conversation on how to let admins know that emails aren't configured. Here's my first attempt. The link goes to: https://docs.sourcegraph.com/admin/config/site_config#email-smtp (which isn't super helpful at the moment either).

I also found https://github.com/sourcegraph/sourcegraph/issues/9017 so maybe we can make improvements to the docs as well?

Test plan