Update protocol support documentation.

- Update old XMPP RFCs to the new one (e.g. 6120)
- Remove the obsolete reference to XEP-0073 and replace it with XEP-0302
- Get rid of the basic/intermediate terms as they are no longer in use in XEP-0302. Use Core and Advanced instead.
- Add XEP-0160, XEP-0198, XEP-0202, XEP-0280
- Correct "Entity Time" to "Legacy Entity Time"
- Correct "Jabber Date and Time Profiles" to "XMPP Date and Time Profiles"