docs: add Docusaurus v3.0 upgrade guide

[slorber]

Motivation

Full upgrade guide to Docusaurus v3, including the final list of breaking changes

https://deploy-preview-9417--docusaurus-2.netlify.app/docs/migration/v3

It's not perfect but should be good enough for the launch.

I think it will be worth it to extract a "MDX troubleshooting" doc page at some point

[netlify]

✅ [V2]

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🔨 Latest commit	2d417a8
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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 84	🟢 97	🟢 92	🟢 100	🟠 89	Report
/docs/installation/	🟠 78	🟢 98	🟢 92	🟢 100	🟠 89	Report
/docs/category/getting-started/	🟢 92	🟢 100	🟢 92	🟢 90	🟠 89	Report
/blog/	🟠 78	🟢 100	🟢 92	🟢 90	🟠 89	Report
/blog/preparing-your-site-for-docusaurus-v3/	🟠 83	🟢 97	🟢 92	🟢 100	🟠 89	Report
/blog/tags/release/	🟠 80	🟢 100	🟢 92	🟠 80	🟠 89	Report
/blog/tags/	🟠 82	🟢 100	🟢 92	🟢 90	🟠 89	Report

[github-actions]

Size Change: +1.09 kB (0%)

Total Size: 1.13 MB

Filename	Size	Change
website/.docusaurus/globalData.json	117 kB	+322 B (0%)
website/build/assets/js/main.********.js	862 kB	+768 B (0%)

ℹ️ View Unchanged

Filename	Size
website/build/assets/css/styles.********.css	113 kB
website/build/index.html	41.4 kB

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[slorber]

@Josh-Cena @OzakIOne merging this right now so that I can release a RC with the final changelog and upgrade guide asap.

Feel free to review and correct any issue in other PRs if you notice anything wrong ;)