Add docs for new identify PUT API

[cbrinicombe13]

I changed the new PUT API to be the 'explicit identification API' and moved the old one to just a 'create user API'. Think that makes more sense.

I also think we should change the SDK method name from .upsert() to .identify() to match the use case better. But i'll need you to change the SDK generation code for that.

[jasontlouro]

This repo should never alter openapi.yml since it will just get overwritten the next time app gets a push to master. The GH Action on it copies its openapi.yml to here. So those changes should be in an app PR

While you're there, to change users.upsert to users.identify I believe it is just changing users_upsert to users_identify here

but obviously that will break RD's code the next time they upgrade the package version

[cbrinicombe13]

I'll create a PR in the app repo as well then can merge both at the same time

[cbrinicombe13]

Related: https://github.com/trophyso/app/pull/170