feat(content-docs): allow omitting enclosing array consistently for category shorthand

[Josh-Cena]

Motivation

Omitting the enclosing array is allowed for the top-level, but not for the children. This PR makes the behavior aligned and eliminates the confusion from some users.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Added two unit tests

[netlify]

✔️ [V2]

🔨 Explore the source changes: 0840627

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/61fc955666cae1000823aeaf

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	66
🟢 Accessibility	100
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	92

Lighthouse ran on https://deploy-preview-6602--docusaurus-2.netlify.app/

[github-actions]

Size Change: +26 B (0%)

Total Size: 772 kB

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	47.2 kB	0 B
website/build/assets/css/styles.********.css	105 kB	0 B
website/build/assets/js/main.********.js	582 kB	+26 B (0%)
website/build/index.html	36.7 kB	0 B

_{compressed-size-action}

[netlify]

✔️ [V2]

🔨 Explore the source changes: dfe5116

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/61fc98aa7673630008db1c02

😎 Browse the preview: https://deploy-preview-6602--docusaurus-2.netlify.app