PYTHON-5643 Add contributor docs for the test.utils_shared.delay function

[caseyclements]

PYTHON-5643

Summary

We have a delay() utility function that we use in testing to trigger a long-running operation on the server.
We should do the following to make it more discoverable and easy to use:

Changes in this PR

1. Add a docstring with an example, calling out that you need to add a document to the collection or the server may decide not to sleep at all.
2. Add a note in the contributing guide pointing out the function's existence and when you might want to use it.

Testing Plan

I've run the docstring examples locally. Are these covered by just docs?

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is the intention of the code captured in relevant tests?
• If there are new TODOs, has a related JIRA ticket been created?

Checklist for Reviewer {@primary_reviewer}

• Where does it feel natural to add this to CONTRIBUTING.md?
• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Have you checked for spelling & grammar errors?
• Is all relevant documentation (README or docstring) updated?

Focus Areas for Reviewer (optional)