fix(docs,theme): auto-generated category index should not display unlisted content

[slorber]

Motivation

Fix #8301

The auto-generated category index should not present content with unlisted: true frontmatter in production mode.

Test Plan

Unit tests

Dogfood: SIMULATE_PRODUCTION_VISIBILITY=true yarn start:website

Test links

• https://deploy-preview-8319--docusaurus-2.netlify.app/tests/docs/tests/visibility => should show 2 subcategories instead of 4 (draft-only + unlisted-only categories are now both filtered)
• https://deploy-preview-8319--docusaurus-2.netlify.app/tests/docs/tests/visibility/some-unlisteds/unlisted-subcategory/ => should show a single listed doc instead of 2

Related issues/PRs

Missing edge case from #8004

[netlify]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟢 91	🟢 97	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 77	🟢 100	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: +141 B (0%)

Total Size: 885 kB

Filename	Size	Change
website/build/assets/js/main.********.js	662 kB	+141 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	70.2 kB
website/build/assets/css/styles.********.css	112 kB
website/build/index.html	40.9 kB

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