docs(website): document custom step implementation

[jcrist]

Once the Step interface solidifies a bit more, we may want to document how users can implement their own Step subclass. We probably won't want to document for external usage for a while (the interface is still in flux and not really user-facing). Docs on the internals may be useful before then for aiding in contributions.

[deepyaman]

#116 (comment) needs to be re-raised and reviewed with feedback incorporated at some point. Raise priority to P1, as @jitingxu1 already did significant work.