fix(content-docs): restore functionality when a category only has index 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

Fix #7348
Fix #7358

Test Plan

I added a few unit tests. These tests were malbehaving prior to the code changes.

I also migrated logger and canary docs to use this pattern so we can dogfood.

Test links

Deploy preview:

• https://deploy-preview-7385--docusaurus-2.netlify.app/community/canary/
• https://deploy-preview-7385--docusaurus-2.netlify.app/docs/api/misc/@docusaurus/logger/

Related issues/PRs

Regression introduced by #6457

[netlify]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 66	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 85	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: +114 B (0%)

Total Size: 811 kB

Filename	Size	Change
website/.docusaurus/globalData.json	52.3 kB	+50 B (0%)
website/build/assets/js/main.********.js	615 kB	+64 B (0%)

ℹ️ View Unchanged

Filename	Size
website/build/assets/css/styles.********.css	105 kB
website/build/index.html	38.8 kB

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