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.

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)