Document the per-box memory layout

It's annoying to have to read the code or "be familiar with uVisor" in order to see what could be easily explained by a picture. We should document the per-box memory layout: what goes where in relation to each other. This means showing where exist the process heap, box context, process stack, box index, and so forth.

The issue names no file or test; start by reading the uVisor memory-layout code and tracing the per-box regions listed in the request. Document their relative placement with a picture and labels covering the process heap, box context, process stack, box index, and other regions identified by the implementation.

Written by the indexing model from the issue text.