Add the "please use latest.html" warning to the s2s spec
Now that we have a release, we should be warning people who try and use the unstable spec as fact.
Federation API
==============
+{{unstable_warning_block_SERVER_RELEASE_LABEL}}
+
Matrix homeservers use the Federation APIs (also known as server-server APIs)
to communicate with each other. Homeservers use these APIs to push messages to
each other in real-time, to retrieve historic messages from each other, and to
