Docs PRs are consistently delayed due to the the fact that we currently
require two approvals to come from a small team of people who are all
very busy.

To mitigate this, we will treat PRs that touch docs similarly to PRs
that touch code. The new behavior is:

1. PRs continue to require two approvals, whether they touch code, docs,
   or both.
2. At least one of these two approvals must come from a code "owner"
   (otherwise known as a "group 1 reviewer"). This becomes the case
   whether the PR touches docs or not.
3. If the PR does not touch docs, then approvals from the docs reviewers
   don't count towards the 2 approval limit.
4. If the PR does touch docs, then the docs reviewers are added to group
   2 reviewers. Their reviews count towards the two review limit, but
   their reviews are not required in order to merge the PR.