add docs for format: sql unit testing

[graciegoheen]

What are you changing in this pull request and why?

Closes #5289

Coming soon (see core PR here) we are introducing a new format for defining mock data for unit tests - format: sql.

This PR adds documentation for that new format.

Checklist

• Review the Content style guide so my content adheres to these guidelines.
• For docs versioning, review how to version a whole page and version a block of content.
• Add a checklist item for anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

Adding or removing pages (delete if not applicable):

• Add/remove page in website/sidebars.js
• Provide a unique filename for new pages
• Add an entry for deleted pages in website/vercel.json
• Run link testing locally with npm run build to update the links that point to deleted pages

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
docs-getdbt-com	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 17, 2024 6:23pm

[graciegoheen]

Let's wait to merge this till after the 17th when we've cut the 3rd beta of 1.8!