Container image virtual registry: documentation (REST API)

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

• Document the REST API endpoints. Similar to https://docs.gitlab.com/api/maven_virtual_registries/.

Note: User documentation will be added in #577291

🛠️ 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 • 18.8 • On track
• Container image virtual registry: performance r... (#549136) • Unassigned • Backlog
• Container image virtual registry: documentation... (#549138 - closed) • Adie (she/her) • 18.6 👈
• Container image virtual registry: additional fe... (#549470) • Unassigned • Backlog