docs(previews): Add an Astro example and a tutorial

[ahuseyn]

We have two examples for HWP Previews plugin, both built with Next.js. Since HWP Previews is a framework-agnostic plugin, we should showcase how it works with a different framework.

Acceptance criteria

• Create an example under the examples folder with Astro and WPGraphQL
• Create a tutorial under the tutorials folder to teach how to create the same example from scratch

[ahuseyn]

Repo had an existing Astro example for the HWP Previews plugin. It was under generic examples folder. I've moved it under plugin folder.