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 page
• How-to: Configure HWP Previews
• How-to: Extend HWP Previews
• Explanation: Core-concepts
• Tutorial: Implement Previews with WPGraphQL
• Tutorial: Implement Previews with REST API
• docs(previews): Add an Astro example and a tutorial #510

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

[ahuseyn]

Created a separate issue for an Astro example and a tutorial: #510