Add an “About API versioning” cookbook page

[gorghoa]

Add a very basic page in core conveying this message:

API versioning is not managed: recommended way is to create a new projet.

I’m a little concerned that this page is quite blunt, I would rather soften the message somehow.

Maybe something like “API versioning is not managed for now, the current recommended way is to create a new project”. But then I would be concerned that it could be interpreted as some broken promise :woman_shrugging:

closes #451

[dunglas]

I’ll add something to support API evolution in 2.3 or 2.4. Can we hold this PR until it is ready?