Reworking of credits/team/etc information locations #746

This is a follow-up to #742 #709.

I merged #742 because I thought getting those credits back in place ASAP was a good idea. But I think it's not ideal that the things that were initially (intentionally) separated have been re-combined in a not ideal way. As I see it there's really 3 different things:

1. The roles/team that is primarily about the current state of the project and who does what as of today
2. The list of everyone who has ever contributed (i.e. "past and present" team)
3. The credits, in the sense of "people who may or may not necessarily be part o the team, but who without which astropy wouldn't be possible or at least not as good"

After #742 these are essentially all squished together on one page. That's definitely better than nothing, but is not ideal. Here's what I think is the ideal state

1 should be where it is - it makes sense that's part of the header because its an imporant quick reference
2 may actually make sense to remove from the main web page but instead link to the relevant part of the latest docs. I think that's more fair in some sense since it doesn't privledge the core package over other coordinated package like it currently does.
3 then belongs as a separate page on its own somewhere. My preference is that it gets added to the "about" dropdown that currently exists. (also probably link to at the bottom of the "team" page since these is all a bit fuzzy). Regardless it must be on the main web page somewhere because some of our funders require this as a condition of funding.

It's a bit debatable whether lifetime contributors end up in 1 or 3 but either way there's still significant content for both.

cc @Cadair