Refactor maven package repository docs

What does this MR do?

• Add instructions how setup automatic maven package deploy via CI/CD
• Document all the different admin options in source and Omnibus.
• Document the API Not needed, it's used only internally.
• Stick to the product naming. Is this "GitLab Packages repository", "Maven packages repository", "GitLab Maven Packages repository", "GitLab Maven repository", something else?

Related issues

https://gitlab.com/gitlab-org/gitlab-ee/issues/5811

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7517

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards