Keep tutorial code in sync with rust-payjoin

[va-an]

Really this thing should have a run check in github CI. I I haven't thought of how to do that exactly, but anything code post on the website really should stay tagged to a specific commit and be reliable basically indefinitely. even better if it's somehow updates in lockstep with the underlying library because it compiles as part of rust-payjoin.

I am sure we are not the first to run into this problem and there are probably 10 solutions available

Originally posted by @DanGould in #139

Code on the site has no mechanism keeping it in sync with rust-payjoin, so tutorials silently stop compiling whenever the library makes a breaking change. Scope is site-wide ("anything code post on the website"), not just receive-payjoin-v2 (which is being fixed in #139).

Proposed solution. Move tutorial code into rust-payjoin/examples/receive-payjoin-v2/ as a workspace member with a path dep on payjoin. Workspace CI then compiles the example on every change, so any breaking API change fails CI in the same PR. The tutorial pins to a rust-payjoin release tag and tells readers cargo add payjoin@X.Y matching that tag. Markdown is updated manually at bump time (one tutorial today, automation not worth it yet).

Plan:

• Update "Receive Payjoin V2" tutorial to current API #139
• Add examples/receive-payjoin-v2/ to rust-payjoin workspace
• Update tutorial's source-code link from payjoin/receive-payjoin-v2 to the new rust-payjoin/tree/v0.X.Y/... location (after next release tag)
• Archive payjoin/receive-payjoin-v2

Needs a cACK from rust-payjoin maintainers before any of the above moves.