DOCS: Add "opt out" of versioning docs

[unclecheese]

Per @chillu 's comment on silverstripe-versioned updating the global schema:

Adding this to all schemas by default encourages the use of previews in your implementation, but also increases the security surface of your API. Given it's non-trivial to opt out of this, I think we need to add more docs on an "opt out config"

We should document how to turn off versioning, since injecting it into all schemas is a bit aggressive.

[chillu]

silverstripe/silverstripe-framework#9769