Add info about env variables to deployment doc

[sarayourfriend]

Description

Noticed this information was missing from here so adding it before I forget again 🙂

Testing Instructions

Read the new docs and let me know if it needs any clarification or other improvements.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
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• My code follows the established code style of the repository.
• [N/A] I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• [N/A] I tried running the project locally and verified that there are no visible errors.

Developer Certificate of Origin

Developer Certificate of Origin

Developer Certificate of Origin
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[github-actions]

Storybook and Tailwind configuration previews: Ready

Storybook: https://wordpress.github.io/openverse-frontend/_preview/1734
Tailwind: https://wordpress.github.io/openverse-frontend/_preview/1734/tailwind

Please note that GitHub pages takes a little time to deploy newly pushed code, if the links above don't work or you see old versions, wait 5 minutes and try again.

You can check the GitHub pages deployment action list to see the current status of the deployments.

[obulat]

LGTM!