docs: suggest using {<...>...</...>} if don't use Markdown in migra…

[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

If you want to migrate complex JSX expressions in MDXv1 + Docusaurus v1/2 to MDXv3 + Docusaurus v3, you can just wrap them with {...}.
You can't use Markdown syntax there (as before), but don't have to care about the extra insertion of <p> elements.
This will help you if you have to migrate documents that have complex JSX expressions that you don't want to touch.

MDX v1

<Foo>
  <Bar>
     <Baz>
        You don't have to care about extra <code>&lt;p&gt;</code> here!<Qux/>
     </Baz>
  </Bar>
  <Corge>
    **no emphasis**
  </Corge>
</Foo>

↓

MDX v2 & 3

{<Foo>
  <Bar>
     <Baz>
        You don't have to care about extra <code>&lt;p&gt;</code> here!<Qux/>
     </Baz>
  </Bar>
  <Corge>
    **no emphasis**
  </Corge>
</Foo>}

Test Plan

https://deploy-preview-10167--docusaurus-2.netlify.app/docs/migration/v3#unintended-extra-paragraphs
https://deploy-preview-10167--docusaurus-2.netlify.app/docs/3.3.2/migration/v3/#unintended-extra-paragraphs

Test links

Deploy preview: https://deploy-preview-10167--docusaurus-2.netlify.app/

Related issues/PRs

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[github-actions]

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[slorber]

makes sense, thanks 👍