docs: v2 docs should discourage the usage of a lower-case MDX component (will not work in v3)

[slorber]

Motivation

In MDX v2, lower-case elements can't be replaced by custom components anymore. Only the upper-case components can.

The following will stop working in Docusaurus v3, so we should start encouraging users to not use lower-case tag names for custom MDX components, even if it still temporarily works

Also did some cleanup that was related to prepare for MDX 2 (for example there's no more MDXComponents/Head component)

Test Plan

preview

Test links

https://deploy-preview-8779--docusaurus-2.netlify.app/docs/markdown-features/react#mdx-component-scope

Related issues/PRs

MDX 2 PR: #8288

[netlify]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 59	🟢 97	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 78	🟢 100	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: 0 B

Total Size: 952 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	86.2 kB
website/build/assets/css/styles.********.css	113 kB
website/build/assets/js/main.********.js	712 kB
website/build/index.html	41.1 kB

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