Fix gamepad examples

[dhalbert]

gamepad.py is no longer in the base library, because the gamepad device was removed from CircuitPython 7.0.0.

• Rename examples/gamepad.py to examples/hid_gamepad.py or similar, because gamepad.py overlaps with the gamepad built-in module.
• Fix or remove the gamepad-based examples.
• Add documentation about how to add a gamepad device to CircuitPython 7.0.0, or refer to a guide.

[dhalbert]

There's now a gamepad example in https://learn.adafruit.com/customizing-usb-devices-in-circuitpython/hid-devices#custom-hid-devices-3096614-9.

[dhalbert]

I think we should rename examples/gamepad.py to examples/hid_gamepad.py, or just discard it. The class name could stay the same.

I don't know whether we should just remove the other gamepad examples, or change them to use hid_gamepad. It seems odd to just discard the code, but they are kind of ancillary if there's no more gamepad.py in the library.

[FoamyGuy]

I'm in favor of keeping them instead of discarding even though gamepad.py is not a part of the library itself. I think it would be a
shame to get rid of it entirely, and I think this is the library that it is most closely related to so it seems like a logical place for that folks looking for something like that would be checking.

I'll make a PR with those changes.