Move federated rule groups prose to its own section

[KMiller-Grafana]

The info on the new feature of federated rule groups is at a 4th-level markdown header within the HTTP API specification docs. Its placement makes the documentation all but impossible to find, since the level means that the header doesn't show up in any table of contents or navigation menu.

Promote this prose to be in its own section.

Once promoted, update the 2.0 release notes, such that the link that currently points to a nearby header within the HTTP API section instead points to the promoted section.