docs: add a troubleshooting page for dangling schemas in an APIExport

[2mind]

Summary

Adds a troubleshooting page for the case described in #188: an APIExport referencing an APIResourceSchema that does not exist.

How to find it — the export itself reports nothing about the dangling entry; the only signal is the consumers' APIBinding (APIExportValid=False, APIResourceSchema "<name>" not found). The page shows how to match that message against the export's schema list, and to use the export's events to tell an agent-managed entry from a hand-written one.

How to recover:

• if the resource should exist — fix the reason the ARS was never created (PublishedResource.status.resourceSchemaName, agent logs); a resource whose CRD cannot be projected leaves the whole export untouched;
• if the resource is not needed — remove the entry from spec.resources (v1alpha2) / spec.latestResourceSchemas (older kcp). Documented as a metadata-only change: no ARS and no synced objects are deleted, only the API stops being served, and the consumers' bindings return to APIExportValid=True on their next reconciliation;
• and the caveat that this does not stick while a PublishedResource for the same group/resource still exists, because the next reconcile adds the agent's own schema name back (and drops the manually added entry for the same group/resource — Warning RemovingResourceSchemas).

Verified against kcp v0.32.x with api-syncagent v0.7.0: with the dangling entry the binding shows Invalid APIExport ... not found, removing the entry restores APIExportValid=True/Ready=True, and the entry does not come back while nothing manages that resource.

What Type of PR Is This?

/kind documentation

Related Issue(s)

Contributes to #188

Release Notes

Document how to find and remove an APIExport resource entry that references a missing APIResourceSchema

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[2mind]

/kind documentation