Adds better type narrowing docs

[sobolevn]

List of changed:

• I've renamed casts into type-narrowing
• I've moved TypeGuard type there, it fits perfectly
• I've added new docs about general type narrowing techniques
• I've added / changed links where it was required

Refs #11014
CC @hauntsaninja

[hauntsaninja]

This isn't generally true, e.g. not even with --strict, I think you need --warn-unreachable for this.

[hauntsaninja]

Could also maybe mention the assert_never trick here though

[hauntsaninja]

This is awesome. I made a bunch of small wording changes. The only substantive comment here is about --warn-unreachable which we need to be a little careful about. E.g., there's some discussion of it here: https://mypy.readthedocs.io/en/stable/common_issues.html#unreachable-code

[sobolevn]

@hauntsaninja thanks a lot for your help! I've added a note about --warn-unreachable

[hauntsaninja]

Thank you!