fix(core): do not recreate ReactDOM Root, fix React warning on hot reload

[slorber]

Motivation

Fix React 18 hot reloading warning:

Initially reported here: #10099 (comment)

Warning: You are calling ReactDOMClient.createRoot() on a container that has already been passed to createRoot() before. Instead, call root.render() on the existing root instead if you want to update it.

Note: this is not a React v18.3 warning, we can also have it in v18.2.

Note: Docusaurus does not have React Fast Refresh support, but we probably should have that. It doesn't look super easy to support 😅

Test Plan

CI + Tests

Local dev, hot reload works for both MD and JS files

Test links

https://deploy-preview-10103--docusaurus-2.netlify.app/

[slorber]

Note to self

I decided to add a new attribute here because it's a mutable variable, unlike window.docusaurus (which gets reassigned on hot reload and is also frozen, so it was complicated to add the mutable root here in practice)

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	9f6789d
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/6634dc5a4eb65f0008c062db
😎 Deploy Preview	https://deploy-preview-10103--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
/	🟠 59	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🔴 49	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 76	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 69	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 63	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 68	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report

[github-actions]

Size Change: +86 B (+0.01%)

Total Size: 1.59 MB

Filename	Size	Change
website/build/assets/js/main.********.js	810 kB	+86 B (+0.01%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/codeTranslations.json	2 B
website/.docusaurus/docusaurus.config.mjs	26.7 kB
website/.docusaurus/globalData.json	91.2 kB
website/.docusaurus/i18n.json	930 B
website/.docusaurus/registry.js	247 kB
website/.docusaurus/routes.js	156 kB
website/.docusaurus/routesChunkNames.json	109 kB
website/.docusaurus/site-metadata.json	2.17 kB
website/build/assets/css/styles.********.css	112 kB
website/build/index.html	38 kB

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