This adds back in the references to private repositories and
provides some refactoring to the Working with repositories
documentation including updating references to the "Central"
registry to Docker.io.
It also:
* Fixes some links and references to Central Index
* Fixes anchors in other files to updated titles in Working with Repositories.
* Renamed Central Index in the remaining places.
* Updated terms documentation to reflect Docker.io
* Updated some Docker Index naming to be consistent.
* Updates menu labels and hyperlinks.
Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
