suggestion: add signed cookie example

[iuioiua]

The @std/http/unstable-signed-cookie is missing an end-to-end example of how the functions can be used together. Perhaps one can be added to Deno by Example.

[MadGanGithub]

Hey @iuioiua,
I'd like to work on this issue. Could you assign me this task?

[espmorfer]

@iuioiua @MadGanGithub is anyone working on this? Should I PR here or at docs repository? I would like to contribute!
Thanks!

[iuioiua]

I don't think @MadGanGithub is working on this. I think Deno by Example is the best place to host this example (info here).