Add 3rd party sign-in docs: Apple Sign-In

[allister-beamable]

One of the 3rd party credentials that Beamable supports is Apple Sign-In, using the "apple" 3rd party identifier in Beamable APIs.

The old docs for using the "apple" auth type are at https://docs.beamable.com/v1.0.0/docs/apple-sign-in-guide

Check whether the old docs are still valid, and either transfer them as-is to new docs or update them to be valid before adding them to new docs.