Add count in zone guide to `supervision` docs

[capjamesg]

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Description

This PR adds a guide on how to count objects in a zone using the PolygonZone function in supervision.

Type of change

• New documentation

How has this change been tested, please provide a testcase or example of how you tested the change?

This change was tested by opening the new docs page to make sure all content rendered properly.

Any specific deployment considerations

N/A

Docs

See above.

[SkalskiP]

Hi @capjamesg 👋🏻

Could you make the changes listed below?

• Use video that can be downloaded using supervision[assets]. You can find an example of docs already doing this here. Let's enable people to go through exactly the same example as we did.
• If you mention a Supervision API element like sv.process_video or sv.PolygonZone in the text, link them. Let's make it as easy as possible for people to find documentation of the blocks they need.
• Show complete code. I don't think we should expect people to be able to put the demo code together themselves, even if we give them all the necessary blocks of code. Our users often ask very basic questions. Once again, Track Objects could be an example.
• Show results. If your code draws a zona in the picture, show what the drawn zona looks like. If you show code that processes the entire video, show the resulting video. Once again, Track Objects could be an example.

[capjamesg]

I have responded to your feedback.