docs: backport #9471, #9692, and #9695 changes in v3.1.1 docs

[tats-u]

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

The changes by those PRs are only applied in Canary and has not been read by many people.

Test Plan

Compare versions between Canary and 3.1.1

https://deploy-preview-9804--docusaurus-2.netlify.app/docs/migration/v3/
https://deploy-preview-9804--docusaurus-2.netlify.app/docs/3.1.1/migration/v3/

https://deploy-preview-9804--docusaurus-2.netlify.app/docs/markdown-features/react/
https://deploy-preview-9804--docusaurus-2.netlify.app/docs/3.1.1/markdown-features/react/

Test links

Deploy preview: https://deploy-preview-9804--docusaurus-2.netlify.app

Related issues/PRs

#9471, #9692, and #9695

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[tats-u]

Do I have to apply this change even to 3.0.1?

[slorber]

Thanks 👍

Do I have to apply this change even to 3.0.1?

You don't "have to", I assume many people will directly upgrade to 3.1 so it's not a big deal to not backport to all 3.x versions.

But I'm fine if you backport more as long as it applies to the version you backport the change to. So, up to you.

Going to move on and merge this directly.

[tats-u]

I will make a choice not to reflect this in 3.0.x. It is just because it takes more work for a smaller impact, though.