Added documentation for docker.io OAuth & Accounts

OAuth docs:
  documented the OAuth authorization flow and how to register your application

Account docs:
  documented getting/updating user profile data
  documented getting/updating user email data

Docker-DCO-1.1-Signed-off-by: Josh hawn <josh.hawn@docker.com> (github: jlhawn)
Updated OAuth docs per arothfusz's comments

Added emphasis of https for OAuth interactions.
Updated example URIs to use HTTPS
Included redirect hostname on authorization page.

Docker-DCO-1.1-Signed-off-by: Josh hawn <josh.hawn@docker.com> (github: jlhawn)
Merge pull request #4319 from jlhawn/docs-docker-io-api

Added documentation for docker.io OAuth & Accounts