Fix missing required RTD configuration key

[Helveg]

ReadTheDocs has made the sphinx.configuration key required, so all RTD builds of sphinx-design are currently failing.

• Example failing build: https://app.readthedocs.org/projects/sphinx-design/builds/28051218/
• Read more: https://about.readthedocs.com/blog/2024/12/deprecate-config-files-without-sphinx-or-mkdocs-config/

[chrisjsewell]

Thanks @Helveg for catching this and for the clear write-up with the failing build link — much appreciated, and sorry it lingered. The sphinx.configuration key has since landed on main and RTD builds are green again, so closing this as completed. 🙏

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