Add a conventions document.

This will contain the style guide (once it's written) and presently
contains the beginnings of a standard example network. The diagram still
needs some work, but all of the important machines are present.
Merge branch 'topic/apeiron/example-network' of https://github.com/apeiron/curriculum into apeiron-topic/apeiron/example-network
Adds the conventions doc to an index to pass, and adds a link from contributions page
whitespace
adds a todo for @apeiron on topography