GAR integration documentation

Create the proper related documentation:

• The list of available project integrations.
• A dedicated page for the project integration presenting:
  • Its relationship with the IAM integration it depends on;
  • Instructions on how to setup the integration;
  • The new menu entry and its pages.
  • The access level required (Guest+ for reads, Developer+ for writes);
  • The available CI/CD env variables;
  • Examples of pulling and pushing an image from CI with the identity: google_cloud feature;
  • Examples of pulling and pushing an image from a local machine.

Page directory:

gitlab/doc/user/packages/google_artifact_registry/index.md

Plan

1. New Google Artifact Registry Project Integration (#425066 - closed)
2. GAR Integration: Custom client class (#425147 - closed)
3. GraphQL: get GAR artifacts from project (#425149 - closed)
4. GraphQL: get GAR artifact details query (#425150 - closed)
5. GAR Integration: Add predefined CI variables (#425153 - closed)
6. GAR integration: frontend menu entry and list o... (#425154 - closed)
7. GAR integration: frontend artifact details page (#425157 - closed)
8. Documentation (👈 this issue)