Describe how Publ works, exactly

[fluffy-critter]

There needs to be a high-level overview of how Publ works, as users seem to be confused about how the template is selected and how content is floated up to it.

It should touch on things like:

• Index and entry templates
• Template overrides for both
• Entry resolution
• Category metadata
• Path aliases (and the difference between alias, mount, and canonical)
• Special templates (login, error handlers)
• Application-level extensions (e.g. Novembeat's submission system)
• Image renditions

Currently there's a lot of low-level usage stuff for things that people already know about but no signposts to point people to from a high level. Bottom-up documentation only makes sense to the implementers of a system.