Update theming docs to ensure number of available themes is not incorrect

[Zenulous]

When working with Primer/React for a project I noticed that nowadays there are far more color schemes on https://primer.style/react/theme-reference than what is denoted in the docs, e.g. dark_high_contrast. It might be better to remove the explicit number of available color schemes from the docs (unless I am mistaken by what a color scheme denotes).

Merge checklist

• [ x ] Added/updated tests
• [ x ] Added/updated documentation
• [ x ] Tested in Chrome
• [ x ] Tested in Firefox
• [ x ] Tested in Safari
• [ x ] Tested in Edge

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[joshblack]

Thanks so much for this! 🙌