Added back OAuth and Accounts API docs pages

Removed a now unused endpoint from the accounts API.
Updated some of the accounts links to point to www.docker.io
as the account signup and resend-email-confirmation links should
no longer point to the index.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Merge pull request #5425 from jlhawn/io-auth-accounts-api-docs

Merged.