feat(mdx-loader): the table-of-contents should display props passed to headings of imported MDX partials

[anatolykopyl]

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

This PR allows the usage of props in partial MDX files.

Test Plan

This PR will be tested via dogfooding.

Test links

Deploy preview: https://deploy-preview-9773--docusaurus-2.netlify.app/
Relevant dogfooding page: https://deploy-preview-9773--docusaurus-2.netlify.app/tests/docs/tests/toc-partials/props

Related issues/PRs

fix #9772

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[anatolykopyl]

I've come up with a crude way of preserving the data passed to props of Markdown partials. For now, this method only works if the prop is a literal, since the prop values are collected at build time.

While not perfect, having only literals is almost always all that is needed when working with partials.

Nonetheless, I think some work can be moved to runtime, to allow the use of any arbitrary expressions.

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/docs/installation	🟠 66	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/category/getting-started	🟠 76	🟢 100	🟢 100	🟢 90	🟠 89	Report
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[slorber]

😅 to be honest I'm not super fan of this

This only works in some cases and does not feel super robust either, and we'll have to handle the users complaining about things it does not support.

I'd prefer to explore another solution: what if we didn't stringify the headings in the first place?

What if we converted headings to JSX instead?

The TOC structure could look like:

export const toc = [
  {
    "value": <>Doc Heading</>,
    "id": "doc-heading",
    "level": 2
  }, 
  ...__tocPartial(props),
]