Move custom directives into Sphinx-click

[Rowlando13]

There are some nice custom directives that allow you to declare and run Click apps that are currently hidden away in pallets-sphinx-theme. I think would go nicely with this package.
They are click-example and click-run. It would be nice to move them here, then we could remove them from the theme. By the way, I am also a maintainer for click.

[Rowlando13]

This shows how they work:

https://kdeldycke.github.io/click-extra/sphinx.html