Section: specialized relayers vs generic relayers

[aadam-10]

Description and context

Documentation (on book.wormhole.com or docs.wormhole.com) should be updated with information about the relayer engine. More context can be found in the linked milestone.

The scope of this task is to create a section that outlines the differences between generic and specialized relayers. It should contain recommendations on when to use which.

Definition of done

Changes are published.