docs: add an example for a simple callable based custom auth

[zyv]

Summary

Added an example of a simple custom auth based on a callable, as mentioned in the discussion below:

#1674

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.