Add user guide for the component library

[ajhillman-ddj]

Ticket owner: @ajhillman-ddj

I am in the progress of writing up a step-by-step user guide for adding components to the component library.

This user guide needs to include:

• How to create the pages (wrapper and component) for a new component.
• How to add, categorise and describe props.
• How to handle bespoke or composite props + a use case for when to do this.
• A checklist for following best practice in writing components.
• How to add playground pages + how to link them to components.