Addressed regression of private repository documentation.

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)
Merge pull request #5414 from jamtur01/privreg

Addressed regression of private repository documentation.