docs: add entity_creation.md doc

[Jonathansumner]

Entity Development & Testing Documentation

Closing #26

• Covers adding to the schema, creating a handler and updating project.yaml to introduce custom entities.
• Discusses testing convention for these custom entities.

[bryanchriswhite]

I think we'll need to add a section about how to add "order by" plugins (in the submodule 🙄) for new entities now that we're supporting custom ordering.

[bryanchriswhite]

Perhaps there's some low-effort change we could make which would allow us to move the custom ordering plugin code out of the subql submodule and into this repo. I think that would improve maintainability.

[bryanchriswhite]

I think it would make sense to add a section about migrations here. We can probably copy, move, and/or reference the DB Migrations section of the README for most of this.