Porting guide to bindings for 3.70 and how to build pre 3.70 versions

[bmbouter]

With 3.70, my understanding is there are breaking changes in the bindings. There are good reasons for them (not recapped here), but my understanding is these changes are best for our community in the long term. There is a roundup of the breaking changes here.

That being said, I think bindings users need clear documentation on two things. This ticket is tracking the creation of those docs.

• How to port bindings usage with a clear example for each breaking change they need to handle.
• How to build the old bindings yourself if you don't want to port right now.