refactor(core): properly code-split NotFound page

[Josh-Cena]

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

While debugging #7238, I noticed our 404 page is not code-splitted properly. Although I think it's still does its work, let's make it more correct.

This still does not fix that issue though, I will need to look deeper into it.

Test Plan

Test links

Deploy preview: https://deploy-preview-7405--docusaurus-2.netlify.app/foo

Related issues/PRs

[netlify]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 86	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 87	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: +57 B (0%)

Total Size: 812 kB

Filename	Size	Change
website/build/assets/js/main.********.js	615 kB	+57 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.3 kB
website/build/assets/css/styles.********.css	106 kB
website/build/index.html	38.9 kB

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