Rewrite TpPresenceMixin introduction
Rewrite TpPresenceMixin introduction

- more complete documentation of how to wire it up to the interfaces
- recommendation to implement both (see fd.o #25019)
- talk about SimplePresence first

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib