Separated user, dev, and design docs.

Renamed: logging.md -> devel/logging.m
Renamed: access.md -> design/access.md
Renamed: identifiers.md -> design/identifiers.md
Renamed:    labels.md -> design/labels.md
Renamed:    namespaces.md -> design/namespaces.md
Renamed:    security.md -> design/security.md
Renamed:    networking.md -> design/networking.md

Added abbreviated user user-focused document in place of most moved docs.

Added docs/README.md explains how docs are organized.
Added short, user-oriented documentation on labels
Added a glossary.
Fixed up some links.
Merge pull request #1851 from erictune/dev_doc

Separate design and user docs