Document the dependency proxy for Maven

📖 Documentation

I think here we need to follow what exists for the dependency proxy for container repositories: https://docs.gitlab.com/ee/user/packages/dependency_proxy/.

I would suggest a new page within the package registry part. That page would be at the same level as https://docs.gitlab.com/ee/user/packages/package_registry/reduce_package_registry_storage.html.

The documentation will need to properly present some caveats/limitations:

• The feature works at the project level only.
• Only pulling operations are supported.
• The order in which the sources are checked for the requested package is:
  1. The project itself.
  2. The external registry.
• It is important to described that if some packages exists in the project when the dependency proxy is enabled or the external registry url is updated, when pulling these package names (+versions), the ones from the project will be returned. In other words, if users create a situation where duplicated packages (same name+version) exists between the project's package registry and the external registry but these packages are not the same (code wise), the one from the project's package registry will be returned.
  • To workaround this limitation, users can delete the package form the project's package registry.

Do this documentation update along with the feature flag removal.

🔭 The Plan

From the investigation:

1. Database model for the dependency proxy settings (#410714 - closed).
2. The Maven dependency proxy API: cache hit path (#410717 - closed).
3. The Maven dependency proxy API: cache miss path (#410719 - closed).
4. The dependency proxy API skeleton for Maven pac... (#410721 - closed).
5. The dependency proxy settings GraphQL API (#410725 - closed).
6. Dependency proxy for Maven: frontend changes (#410726 - closed).
7. (Option) Dependency proxy for Maven: the prefill option (#410730 - closed).
8. The documentation changes and feature flag removal. (👈 this issue)