[DOCS] Move and expand saved object conflict resolution example

### Description

As part of the migration of API reference information to https://www.elastic.co/docs/api/doc/kibana, we're retiring the old manually-maintained pages. However, per https://github.com/elastic/kibana/pull/195333#discussion_r1802715432, the example about a failed attempt to copy saved objects with conflict errors needs more explanation than seems appropriate for the new format.

Therefore, IMO the https://www.elastic.co/guide/en/kibana/8.15/spaces-api-copy-saved-objects.html#spaces-api-copy-saved-objects-example-3 example should instead be moved and expanded within the narrative docs. For example, perhaps https://www.elastic.co/guide/en/kibana/8.15/managing-saved-objects.html#managing-saved-objects-copy-to-space should be expanded to cover the types of errors you might encounter (in both the UI and API, I'm assuming) and how to resolve them. Or perhaps that's more appropriate for a separate troubleshooting page.

### Resources

N/A

### Which documentation set does this change impact?

Elastic On-Prem only

### Feature differences

The `api/spaces/_copy_saved_objects` endpoint doesn't seem to exist in the list of Kibana Serverless APIs at this time (per https://www.elastic.co/docs/api/doc/serverless/group/endpoint-saved-objects) though I haven't tested whether the UI can nonetheless encounter conflict errors.

### What release is this request related to?

8.16

### Collaboration model

The documentation team

### Point of contact.

**Stakeholders:**

@elena-shostak

## Action

Copy over error content from old API docs to the https://www.elastic.co/docs/explore-analyze/find-and-organize/saved-objects page