Spec reorganization pt 1

Related to opencontainers#126

The following sections have been removed:
- Changes
- Historical Context (already in reorg)
- Overview / Listing Repositories (unused catalog API)

The following sections have been moved into commented reorg:
- Notational Conventions ---> Definitions / Document Language
- Overview / Errors ---> HTTP API / Error Codes
- Overview / API Version Check ---> Conformance / Workflow Categories / Pull
- Overview / Pulling An Image ---> Conformance / Workflow Categories / Pull
- Overview / Pushing An Image ---> Conformance / Workflow Categories / Push
- Overview / Listing Image Tags ---> Conformance / Workflow Categories / Content Discovery
- Overview / Deleting an Image ---> Conformance / Workflow Categories / Content Management
- Detail / Errors ---> HTTP API / Error Codes

Signed-off-by: Josh Dolitsky <393494+jdolitsky@users.noreply.github.com>