Include PPX by example to the manual

[NathanReb]

@pedrobslisboa has been working on a great piece of documentation for ppx authors: https://github.com/pedrobslisboa/ppx-by-example.

Following a short discussion on the discuss announcement post they agreed to integrate it directly into ppxlib's manual as it would make sense to centralize all PPX related documentation and guides here.

I'm opening this issue so we can track progress on this and figure out the details on how to best integrate this.