.readthedocs.yml: Always build latest doc/releases pages

.readthedocs.yml: Always build latest doc/releases pages

We don't backport PRs merged into doc/releases.  Therefore, when one browses to an older Ceph release version on docs.ceph.com (e.g., https://docs.ceph.com/en/pacific/), the information is out of date at best.

The doc/releases page is only accurate if browsing https://docs.ceph.com/en/latest/, for example.

So this post_checkout command will make sure we've checked out doc/releases from main before building and publishing.

Signed-off-by: David Galloway <dgallowa@redhat.com>
(cherry picked from commit 055fe1f825b0629b7685d6d3d4d629ffc37a2d7c)