Add link to props.css to the documentation

[HiDeoo]

What kind of changes does this PR include?

• Changes or translations of Starlight docs site content

Description

As discussed on Discord, this PR adds a link to the list of Starlight CSS custom properties in the Custom CSS styles section of the docs.

This should be a good starting point for users looking for them until we get a more complete page for them considering that this link has already been shared a few times on Discord and I assume many people looked for it in the repo after not seeing it in the docs.

[changeset-bot]

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Latest commit: 8bedd3e

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[dreyfus92]

LGTM ✅

[delucis]

Thanks for this @HiDeoo! Going to tweak this slightly and make the link text a little more descriptive for anyone discovering it via assistive technology.