Add guide for using Request Header and improve type hints usage

[skokado]

Hi, I'm very greateful this project!

I ran into a few issues when trying to use custom headers.

First, the function argument my_header defined as follows didn't work no matter what I did.

from ninja import Header

@api.get("")
def events(request, my_header: Header[str]):
    return {"received": my_header}

I got a 422 status code.

$ curl -H 'my_header: abc' localhost:8000 
{
  "detail": [
    {
      "type": "missing",
      "loc": [
        "header",
        "my_header"
      ],
      "msg": "Field required"
    }
  ]
}

To resolve this, I had to use an alias to separate the header name with hyphens.

@api.get("")
def events(request, my_header: str = Header(alias="my-header"):
    return {"received": my_header}

$ curl -H 'my-header: abc' localhost:8000
{"received": "abc"}

I also discovered a "mypy cheat" block in ninja.params.
This causes Header to always be annotated in the editor, which causes Header() to issue a warning.
I'm not entirely sure why "mypy cheat" is necessary today, but it at least seems to work.

I've also added documentation based on the issues I encountered.

Personally, I think it would be better to expect similar usage for parameters like Query and Path in ninja.params, but I won't make any changes in this PR.

[skokado]

A little more information:

I added the hyphenated alias example because I was considering integration with OpenAPI.

By default, the header name would be in snake case, which would result in a 422 response, which I thought might confuse developers.

When alias not set for argument my_header, the header name in OpenAPI also would be my_header