Reading across collections globally

[rofrankel]

📑 Proposed changes

Add guidance for supporting reading across collections with different resource hierarchies.

Note: this is downstream of https://github.com/aep-dev/aep.dev/pull/164.

🍱 Types of changes

What types of changes does your code introduce to AEP? Put an x in the boxes that apply

• [x] Enhancement
• [ ] New proposal
• [ ] Migrated from google.aip.dev

📋 Your checklist for this pull request

Please review the AEP Style and Guidance for contributing to this repository.

General

• [x] Basic Guidance is met.
• [x] Ensure that your PR references AEPs correctly.

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• [ ] Ensure that the PR is editable by maintainers.
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• [ ] Ensure that Document structure guidelines are met.

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