Serve hugo-generated page for xmpp.org/rfcs

[cal0pteryx]

Currently https://xmpp.org/rfcs redirects to https://xmpp.org/rfcs/, which is a directory listing.

In xsf/xmpp.org#975 I built a new index page for xmpp.org/rfcs. In order to make this work correctly, we would need a server direction for xmpp.org/rfcs. Since xmpp.org serves some RFC files itself (e.g. https://xmpp.org/rfcs/rfc3920.html), and it should continue to do so, we would need an exact match directive for that (and only that URL), I think.

[mwild1]

Done. Actually note that the hugo container is redirecting /rfcs -> /rfcs/ so it wasn't enough to just make that URL work.

The current rule in place will proxy /rfcs/ but not /rfcs/*. The only quirk here is if someone linked to /rfcs/index.html they'll get a 404 instead of seeing the index.html served by Hugo. I think this is fine...

Hopefully nothing else broke (I did a quick test), but I guess we'll hear if it did :)