docs: update docusaurus-plugin-sass instructions

[erickzhao]

Motivation

The docusaurus-plugin-sass plugin has sass as a peer dependency, so you
need to install that package as well, as per their README.

Have you read the Contributing Guidelines on pull requests?

Yes!

Test Plan

N/A (docs change)

Related PRs

N/A

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[netlify]

✔️ [V2]
Built without sensitive environment variables

🔨 Explore the source changes: d2d5833

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😎 Browse the preview: https://deploy-preview-5352--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	56
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5352--docusaurus-2.netlify.app/

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[slorber]

thanks 👍