fix(mdx-loader): improve mdxJsxTextElementToHtml

[slorber]

Motivation

MDX headings are converted to html string to generate toc items.

This is a quite hacky and unreliable process that we already used in MDX v1. Now that MDX v2 is more strict, we have to implement more of this hacky process ourselves.

This PR slightly improves the process by allowing users to pass a className attribute to heading JSX elements.

This is used on a few websites, notably the React-Native website:

### <div class="label required basic">Required</div> **`data`**

https://reactnative.dev/docs/flatlist#required-data

Letting the className pass-through the TOC heading serialization is important to let users customize how it renders on the TOC.

It is possible that supporting other attributes become useful in the future. If it's the case I'd prefer you to let me know so that we can study that on a case-by-case basis.

Ultimately, this remains a workaround. We should probably emit real JSX nodes instead of HTML strings to render the TOC, but that's a more complex refactor.

Test Plan

CI

Test links

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

Related issues/PRs

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	1d26a47
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/64e8db788957f200089d2ccd
😎 Deploy Preview	https://deploy-preview-9262--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 78	🟢 97	🟢 92	🟢 100	🟠 89	Report
/docs/installation	🟠 74	🟢 98	🟢 92	🟢 100	🟠 89	Report

[argos-ci]

The latest updates on your projects. Learn more about Argos notifications ↗︎

Build	Status	Details	Updated (UTC)
default (Inspect)	🧿 Changes detected (Review)	1 change	Aug 25, 2023, 5:10 PM

[github-actions]

Size Change: 0 B

Total Size: 1.11 MB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	117 kB
website/build/assets/css/styles.********.css	113 kB
website/build/assets/js/main.********.js	837 kB
website/build/index.html	41.3 kB

_{compressed-size-action}