Container image virtual registry: documentation (usage of feature)

Add all the related document for the container image virtual registry.

• User documentation to use the feature. It should be located under https://docs.gitlab.com/user/packages/virtual_registry/ in a dedicated tab. This should present how to create virtual registry objects (registry, upstreams and cache entries) but also how to configure the clients to pull images through the virtual registry.

Note: REST API docs will be added in #549138

🛠️ Implementation plan

• Container image virtual registry: database models (#548783 - closed) • Adie (she/her), Radamanthus Batnag • 18.5 • Needs attention
• Container image virtual registry: cache entry s... (#549103 - closed) • Radamanthus Batnag • 18.5 • On track
• Container image virtual registry: upstream serv... (#549104 - closed) • Adie (she/her) • 18.4
• Container image virtual registry: handle file r... (#549134 - closed) • Adie (she/her) • 18.6
• Container image virtual registry: manage APIs (... (#548794 - closed) • Adie (she/her) • 18.5 • On track
• Container image virtual registry: push/pull end... (#549131) • Radamanthus Batnag • 18.6
• Container image virtual registry: performance r... (#549136) • Unassigned • Backlog
• Container image virtual registry: documentation... (#549138) • Adie (she/her) • 18.6 👈
• Container image virtual registry: additional fe... (#549470) • Unassigned • Backlog