Automatically create the necessary directories before writing compiled files in the assets plugin

Previously, the assets plugin would fail if trying to write a file
and the directory did not already exist.

While here, remove documentation for :compiled, since it should
not be used by the user.
Add a conventions guide with a brief overview of the conventions for Roda applications

Hopefully by having a set of official conventions for Roda
applications, the Roda ecosystem will evolve such that most
Roda applications follow these conventions, and end up with
a similar structure, so that users can start working on an
existing Roda app and be able to get up to speed quickly.