[DOC] Refactor/Rename User & Programmer Manuals

[teald]

With more substantial developer documentation, and an increasingly non-intuitive documentation tree, the User Manual and Programmer's Manual need to be reorganized a bit. Right now, it's unclear without investigation which documentation is immediately appropriate for a user (e.g., from the names/front page matter alone). There's also some overlapping information across both of the manuals and the rest of the documentation.