fix(docs): the _category_.json description attribute should display on generated index pages

[bharateshwq]

Motivation

This pull request adds support docs _category_.json description attribute.

This description attribute will display on the autogenerated index page.

Fix #7818

Test Plan

https://deploy-preview-10324--docusaurus-2.netlify.app/tests/docs/category/tests

Thi works for category of autogenerated sidebar slice, using _category_.json description:

This also works for manually defined sidebar category with a description

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	Report
/	🟠 50	🟢 98	🟢 96	🟢 100	Report
/docs/installation	🟠 53	🟢 97	🟢 100	🟢 100	Report
/docs/category/getting-started	🟠 74	🟢 100	🟢 100	🟠 86	Report
/blog	🟠 67	🟢 96	🟢 100	🟠 86	Report
/blog/preparing-your-site-for-docusaurus-v3	🔴 48	🟢 92	🟢 100	🟢 100	Report
/blog/tags/release	🟠 69	🟢 96	🟢 100	🟠 86	Report
/blog/tags	🟠 75	🟢 100	🟢 100	🟠 86	Report

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[slorber]

LGTM thanks

Did some cleanup but it's working 👍