Chore: Integrate HWP Toolkit docs into the docs site

[colinmurphy]

Overview

As part of our roadmap - https://github.com/wpengine/faustjs/blob/canary/ROADMAP.md

We need to Integrate initial developer docs for Headless WP Toolkit

This will be defined as part of #487

• chore(previews): Validate examples and improve scripts #509
• Include plugin-specific examples to the docs

[ahuseyn]

@colinmurphy added this task to cover explanation and how-to docs: #502