Describe build steps

[jmooring]

Need to find a place for this to live in the documentation.

Notes from bep...

In general, Hugo's build is:

1. Process: Read files, insert pages and resources into a set of prefix trees. We do this in parallel (last item wins), so the order is not entirely deterministic.
2. Assemble: Apply cascades, build taxonomies, calculate dates etc.
3. Render: We render each site and output format in serialized order, but the pages within e.g. a site is rendered in parallel.

So, in some sense, the user can "define rendering order", but not the "processing order".

Maybe create a new page in the About section: Architecture?