Clarify difference between "private registry" and "custom registry" on Software docs

[jwitz]

Issue Type

Inaccurate, misleading, or out-of-date information

Links to Affected Docs

https://docs.astronomer.io/software/custom-image-registry

https://docs.astronomer.io/software/kubepodoperator#pulling-images-from-a-private-registry

The Issue or Context

As mentioned in docs, a "private registry" is any alternative registry that contains an image that can be run using the KE or KPO.

A "Custom registry" is a Docker registry that specifically hosts images for Deployments. We should clarify that these are separate entities with very different use cases.

Fix or Proposed Change

No response

Additional Notes

No response

Required Reviewers

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