add elastic scaling MVP guide (#4663)

Resolves https://github.com/paritytech/polkadot-sdk/issues/4468

Gives instructions on how to enable elastic scaling MVP to parachain
teams.

Still a draft because it depends on further changes we make to the
slot-based collator:
https://github.com/paritytech/polkadot-sdk/pull/4097

Parachains cannot use this yet because the collator was not released and
no relay chain network has been configured for elastic scaling yet