Add autogpt example

[Maknee]

Adds a very basic version of autogpt as a more complex example

[zhudotexe]

Cool! Since the docs don't go into too much detail about the impl, you might want to just remove the code block and link to the source.

Alternatively, we could have like a "see also" section at the bottom of advanced.rst for more involved examples:

See Also
--------
Our examples also showcase cool advanced use cases with kani, some of which are highlighted here.

AutoGPT
^^^^^^^
[desc, link, example output]