docs: the charm relation interfaces repo info is out of date

[tmihoc]

Please describe the question or issue you're facing with "How to manage relations - Ops documentation".

In https://documentation.ubuntu.com/ops/latest/howto/manage-relations/#declare-the-relation-endpoint there's a note about the charm-relation-interfaces repo, but when I go there there's a banner that says:

This repository was archived by the owner on Nov 26, 2025. It is now read-only.

(1) Feels like this needs to be updated.

(2) What's the new process?

Thank you very much!

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Reported from: https://documentation.ubuntu.com/ops/latest/howto/manage-relations/