[codewarnab]

Which issue does this PR close?

• Part of Move code in user_defined_plan.rs to the extending-operators doc #15774.

Rationale for this change

The TopK example in user_defined_plan.rs shows how to add a custom operator to DataFusion. Users who read the Extending Operators guide cannot find it there. This PR adds a short walkthrough of the example to that guide.

What changes are included in this PR?

• A new "A custom TopK operator" section in extending-operators.md. It covers:
  • the custom logical node, TopKPlanNode
  • the optimizer rule that adds the node to the plan
  • the physical operator that runs it
  • the limits of the example
• A small update to the intro, so that it covers both examples.

The µWheel section, the example code, and its invariant tests are unchanged. A follow-up PR will move the invariant tests into focused unit tests. That PR will also update the link in this section if the example moves.

What is the testing strategy for this PR?

This PR changes only documentation. The snippets and descriptions match the current code in user_defined_plan.rs.

Are there any user-facing changes?

Yes. The library user guide has a new section. There are no API changes.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has insufficient remaining quota. Try requesting a review at a lower effort level.

[codewarnab]

Hi @alamb, a gentle ping on this when you have time. It’s a small docs-only change (one file) that adds the TopK walkthrough, kept small as you suggested on #15774. CI is green. Happy to adjust anything. Thanks!