Improving mirror list page

[nirik]

From downstream report at: https://pagure.io/fedora-infrastructure/issue/8868
NOTE: orig reporter doesn't use github, so please use the downstream report or direct email to ask them any questions.

"Hello,

I see two issues with the current mirror list at
https://mirrormanager.fedoraproject.org/mirrors/Fedora/31/x86_64

The first one is due to the table of mirrors not having horizontal separators
and having all items centered vertically. I propose that either horizontal
separators are added or that the items are aligned to the top of their rows.

The second issue is that there are no instructions linked on these pages on
how to use these mirrors. For quite a while I was putting the base url into
Anaconda and wondering why it did not work. There is no explanation that we
must manually craft the second part of the URL (so that it contains repodata/)"

So, this is to improve the formatting on the page and also to add some documentation about how to use the mirror urls on that page. I think we should note strongly to use metalink normally.