feat: report orphan pages

[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 Warning or error for orphan docs #2875) and the maintainers have approved on my working plan.

Motivation

Currently just a POC

Test Plan

See deployment log:

[WARNING] Orphan pages found: 
- /__docusaurus/debug/config/
- /blog/archive/
- /search/
- /tests/blog/archive/
- /tests/pages/layout-no-children/
- /tests/docs/folder with space/doc 1/
- /tests/docs/folder with space/doc 2/

The /blog/archive page being orphaned is unfortunate. We need to fix it. Therefore I don't think we can merge it until #5701 and #5461 are both addressed.

Test links

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

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
/	🟠 73	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 85	🟢 100	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: +61 B (0%)

Total Size: 801 kB

Filename	Size	Change
website/build/assets/js/main.********.js	603 kB	+61 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.6 kB
website/build/assets/css/styles.********.css	106 kB
website/build/index.html	38.9 kB

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