Add how-to doc for defining a new Protocol

[dotsdl]

As one of the extensible points (#499) in gufe, we should include a how-to doc on how to define entirely new Protocols. This should highlight:

• subclassing Protocol and ProtocolResult
• defining custom ProtocolUnit subclasses
• practical advice for how to structure Protocol.create() to yield a ProtocolDAG that is parallelizable
• interfaces between Components that are useful for Protocols, such as methods for going to and from openmm, openff, and rdkit objects

This should reference the corresponding concept doc on Protocols, which covers what Protocols do and how they are structured.