UI documentation

[Rod Widdowson]

> I think probably the most natural place is under the Authentication topic
> in a separate page. Most of the uses of this are going to be logout.
> 
> Consent of course being the other, but we don't have that documented yet
> and we can always link to it.

I made a first attempt at laying this out here 

https://wiki.shibboleth.net/confluence/display/IDP30/VelocityProperties

Comments welcome, before I add too much more.  

I think this page may get very big very quickly at which point each Velocity
Property of interest will need to go into its own page.  I do not see us
writing much, if anything about things like the profileRequestContext or the
Request.

R