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)
Upstream-commit: cb003686e4
Component: cli

components/cli/docs/sources/reference/commandline/cli.md

@@ -694,10 +694,10 @@ Pull an image or a repository from the registry
     Usage: docker pull NAME[:TAG]
 
 Most of your images will be created on top of a base image from the
-Docker Index ([https://index.docker.io](https://index.docker.io)).
+[Docker.io](https://index.docker.io) registry.
 
-The Docker Index contains many pre-built images that you can
-`pull` and try without needing to define and configure your own.
+[Docker.io](https://index.docker.io) contains many pre-built images that you
+can `pull` and try without needing to define and configure your own.
 
 To download a particular image, or set of images (i.e., a repository),
 use `docker pull`:
@@ -1067,7 +1067,7 @@ It is used to create a backup that can then be used with
 
 ## search
 
-Search the docker index for images
+Search [Docker.io](https://index.docker.io) for images
 
     Usage: docker search TERM
 
@@ -1075,8 +1075,8 @@ Search the docker index for images
      -s, --stars=0: Only displays with at least xxx stars
      -t, --trusted=false: Only show trusted builds
 
-See [*Find Public Images on the Central Index*](
-/use/workingwithrepository/#searching-central-index) for
+See [*Find Public Images on Docker.io*](
+/use/workingwithrepository/#find-public-images-on-dockerio) for
 more details on finding shared images from the commandline.
 
 ## start