Adds docs for TypeGuard type, #10591

[sobolevn]

I am writting an article about TypeGuards and I've noticed that mypy does not document how TypeGuards work.

So, here it is!

It has several examples from the PEP itself and several custom ones.
I have tried to cover all corner-cases that I am aware of, but possibly there are other details that I've missed.

I would appreciate your feedback! 👍

Closes #10591

[JelleZijlstra]

Thanks! I have a couple of wording suggestions.

Also, I'm not sure the "Casts" section is the right place for these docs. "More types" is the traditional place for advanced type system features. I'm curious to hear others' opinions.

[sobolevn]

@JelleZijlstra thanks! I thought that "casts" is the closest to type guards. I can move it, no problem.

[hauntsaninja]

Thank you for this! Fixed a couple small nits

[sobolevn]

@JelleZijlstra @hauntsaninja so, should I move this to "Advanced types"? Or is "Casts" the proper place? 🙂

[hauntsaninja]

I'd probably put it in "More types" too, but don't feel strongly.

[sobolevn]

Moved! 👍

[sobolevn]

@hauntsaninja @JelleZijlstra friendly ping 🙂

[hauntsaninja]

Thanks again! :-)