Add documentation for dependency proxy feature

See merge request gitlab-org/gitlab-ce!28096

(cherry picked from commit f4dfbb68)

516344b9 Add documentation for dependency proxy feature
084fee73 Merge branch 'master' into docs/dependency-proxy-ce
14b9e8a5 Copyedit Dependency Proxy docs
10811da0 Link dependency proxy to the group index page
900d40ab Add example of a namespaced Docker image
d7e2b4a7 Add link to Puma for source installations
18e68c5e Merge branch 'master' into docs/dependency-proxy-ce
ac62bff2 Add note about data retention and fix heading