Move send and recieve doc tests to payjoindevkit.org

[DanGould]

@jbesraa discovered the Documentation tests are broken because they're not written as Documentation tests should be

Either the tests should be moved out of the file's documentation and into a PDK example "Send and Receive Payjoin in Rust" like this one from LDK or be written according to documentation tests standards so they can be run in place. The main benefit of using documentation tests is that running them in place allows us to catch errors, but it seems we already have explicit integration tests to do that.

My preference would be a tutorial that we can link to from the docs. Here's the article from LDK in payjoindevkit.org repo that can be replaced and linked to by adding a line here