docs: changed actions/checkout version from 2 to 3

[cm-igarashi-ryosuke]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

actions/checkout@v2 is deprecated because it depends on Node.js12, so it has been modified to v3.

Test Plan

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

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✅ [V2]

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🔨 Latest commit	197b83d
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😎 Deploy Preview	https://deploy-preview-8390--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
/	🟢 95	🟢 97	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 79	🟢 100	🟢 100	🟢 100	🟢 90	Report

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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[slorber]

Thanks

There are other refs to v2 in this file, can you update all refs please?

Also can you backport this to older v2.x versions too?