docs(content-docs): add api doc for displayed_sidebar front matter

[slorber]

Motivation

Missing existing API doc for feature implemented in #5782

cc @Josh-Cena

Test Plan

preview

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	5531f6f
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😎 Deploy Preview	https://deploy-preview-8146--docusaurus-2.netlify.app
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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 67	🟢 98	🟢 100	🟢 100	🟠 80	Report
/docs/installation	🟠 78	🟢 100	🟢 100	🟢 100	🟢 90	Report

[Josh-Cena]

LGTM

[github-actions]

Size Change: 0 B

Total Size: 814 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.1 kB
website/build/assets/css/styles.********.css	112 kB
website/build/assets/js/main.********.js	609 kB
website/build/index.html	40.8 kB

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