chore: Add docs for HWP Previews

[colinmurphy]

Overview

We need to add the following docs unders /docs/plugins/hwp-previews (see #470) and also update the nav.json too for the sidebar.

• Index
  • Tutorial
    • Faust Tutorial with WPGraphQL example
    • REST tutorial
    • Astro HWP Previews Example
  • How To Guides
    • Configurue HWP Previews
    • Extend HWP Previews

We should also reference the docs in the README and ensure these are covered and migrated too.