[docs] Add recipe on using a theme

[marcysutton]

When I was working on #20113, I noticed that the only "Using a Theme" recipe requires you to use a starter. For people wanting to know how to install and use a theme, starters create extra cognitive overhead: "what if I want to install a theme that isn't part of a starter?" We've made changes in the Themes docs in the past month or two, but the recipes weren't updated as part of that effort. This issue will be satisfied when there is a recipe on Using a Theme Directly (or something to that effect).

See the Using Themes docs for inspiration: https://www.gatsbyjs.org/docs/themes/using-a-gatsby-theme/

A recipe should be a short, actionable piece of documentation that shows a Gatsby learner how to complete a task. Follow the Gatsby Style Guide and Docs Templates for instructions on how to create a recipe PR that will be accepted.

Related to #20113, which moves recipes to individual pages. It must be merged first.

[github-actions]

Hiya!

This issue has gone quiet. Spooky quiet. 👻

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