Document deriv, update/correct docs for rate and delta.

License cleanup

Make documentation for absent() not, uhm, absent

Add an FAQ

Move the rules over to the querying section, that seems more natural.

Add some guidance on recording rules naming.

Also cover how to correctly handle means when aggregating.

Expand console template docs.

Explain the general structure, prom_query_drilldown and graph library.

Expand Pushgateway docs with Java example.

Be consistent in saying 'Pushgateway'.

Copy template docs over from the wiki.

Add links to client code for the pushgateway.

Make getting started link on frontpage go to Getting started section.

Put exposition formats at the end of instrumenting, as it's the least relevant
to normal users.

fix a broken link