Documents for using custom entities in KIC

[randmonkey]

Is there an existing issue for this?

• [X] I have searched the existing issues

Does this enhancement require public documentation?

• [X] I have added an Acceptance Criteria item for adding and/or adjusting public documentation (if applicable)

Problem Statement

Split from #4556 custom entities. We need to add the guide to introduce how to use Kong custom entities in k8s when KIC supports them.

Proposed Solution

• Add a guide page in KIC docs to introduce how to setup a plugin (like degraphql) using custom entities
• Introduce the KongCustomEntity CRD in references

Additional information

No response

Acceptance Criteria

• [ ] KIC docs includes a guide page for using custom entities
• [ ] KIC docs includes reference of KongCustomEntity CRD.