Feature Request: Developer-centric guide to creating an overlay

**Is your feature request related to a problem? Please describe.**

As a developer who isn't a core maintainer of Overlayed, I'd like to be able to follow along with a quick guide to creating an Overlay, using my language (within reason - ie: `typescript` and `javascript`) and toolchain of choice.

**Describe the solution you'd like**

A written document outlining the steps to creating an overlay (what will likely be called `plugins` by the core team) with variations for different approaches. Referencing the built-in plugins would be valuable as well.

Start by reading the repository's existing built-in plugins and the surrounding overlay implementation to identify the steps and supported approaches. The work is complete when a written developer guide explains how to create an overlay, includes TypeScript and JavaScript variations where practical, and references the built-in plugins.

Written by the indexing model from the issue text.