chore(docs): Update sourcing-from-prismic

[pvpg]

Hello Gatsby team, I'm opening a new PR for this Gatsby and Prismic page.

1. I added links to our new documentation.
2. Some minor changes to match the correct plugin configuration
3. Minor change to the gatsby-node query to generate the pages using the post.url value from the query.
4. And, a validation before saving the query responses. e.g., if (!data) return null

Let me know if you have any comments.
Thanks