Add documentation on FAST templating

[janechu]

Pull Request

📖 Description

This adds some code docs describing various aspects of the html templating system with regards to bindings and applying events to HTML, it also describes how hydration works within the templating system system.

Mermaid diagrams have been added for further elucidation.

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.