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

[zyv]

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Summary

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

https://github.com/encode/httpx/discussions/1674

Checklist

• [X] 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.
• [X] I've updated the documentation accordingly.