docs(integrations): add framework README

[miguelpeixe]

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Changes proposed in this Pull Request:

Adds a developer-facing README for the Reader Activation Integrations framework, modeled on includes/data-events/README.md. Covers the framework purpose, file map, built-in esp integration, the registration flow, required and optional methods on the Integration base class, settings declaration, push/pull pipelines, per-integration data event handlers, ActionScheduler groups, health checks, My Account endpoints, frontend reader registration, and a hooks/storage reference.

This is documentation only — no behavioral changes.

Closes NPPD-1082.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[Copilot]

Pull request overview

Documentation-only PR adding a developer-facing README for the Reader Activation Integrations framework, modeled on the data-events README.

Changes:

• New includes/reader-activation/integrations/README.md describing framework purpose, file map, registration flow, push/pull pipelines, settings, data event handlers, ActionScheduler groups, health checks, My Account endpoints, and frontend registration.

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[github-actions]

Hey @miguelpeixe, good job getting this PR merged! 🎉

Now, the needs-changelog label has been added to it.

Please check if this PR needs to be included in the "Upcoming Changes" and "Release Notes" doc. If it doesn't, simply remove the label.

If it does, please add an entry to our shared document, with screenshots and testing instructions if applicable, then remove the label.

Thank you! ❤️