docs: Production requirements

[tsibley]

Motivated by our work with CDC for them to be able to internally host nextstrain.org, but this documentation will also be useful for ourselves.

We can adapt/amend it later to account for any "Nextstrain Groups Server" (i.e. just the Groups bits of nextstrain.org) that we end up creating.

Related-to: https://github.com/nextstrain/private/issues/94

Checklist

• [x] Preview of page looks good
• [x] Checks pass

[tsibley]

This is based on https://github.com/nextstrain/nextstrain.org/pull/733 so I'll wait for that to merge before merging this.

[tsibley]

There's still a gap in documenting how these are configured for our actual nextstrain.org instance, but that's beyond this PR.

True, there's not documentation of our actual deployment structured like this new page, but most of these bits are documented somewhere. The infrastructure page at least mentions many of them. The sessions page discusses Redis in that context too. The Terraform configurations and corresponding config files also detail specifics.

[tsibley]

This is based on #733 so I'll wait for that to merge before merging this.

Actually, since I'm making a few minor fixups to #733, I'll first merge this into that (and then that into master).