Add a pyjanitor interoperability example

[JohnnyWilson16]

Summary

Add a small docs example showing how freshdata can run before or after pyjanitor
in a pandas workflow.

Acceptance Criteria

• The example uses a small inline DataFrame.
• The docs explain when to use freshdata versus pyjanitor.
• mkdocs build --strict passes.

From issue-backlog.md §3.

[WilliamK112]

I would like to take this. I’ll add a small optional PyJanitor interoperability example using one inline DataFrame and demonstrate both useful orderings: PyJanitor first for explicit column-shape transformations followed by FreshData quality repair, and FreshData first followed by a targeted PyJanitor transformation. I’ll document the responsibility boundary, keep PyJanitor out of core dependencies, link the example from the existing examples index, and validate the script plus mkdocs build --strict.