docs: Outdated Reference to `charm-relation-interfaces`

[wctaylor]

Please describe the question or issue you're facing with "Charm-relation-interfaces - Ops documentation".

The Ops docs have a page about charm-relation-interfaces with a link to the charm-relation-interfaces repo, but that repo has been archived and instead refers people to the Charmlibs documentation and charmlibs repo. The Ops documentation should probably be updated accordingly.

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Reported from: https://documentation.ubuntu.com/ops/latest/explanation/charm-relation-interfaces/

[tonyandrewmeyer]

@james-garner-canonical what do you think is best here? Remove the page entirely? Rework it to be about charmlibs?

[james-garner-canonical]

I've been meaning to rework this one for a while, thanks for the issue report.