Separate design and user docs

Moved design documentation into a subdirectory.
Created some new user-focused documentation, and a glossary.
Partially resolves #1813
I'm not sure about binary -- it's still a string, not bytes.

Also, please emphasize the intended usage of annotations.

In the case of annotations, something like:

A key/value pair recording non-identifying metadata associated with an object, such as provenance information.