fix(content-docs): use category index convention to infer document title

[MarkShawn2020]

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• [x] I have read the Contributing Guidelines on pull requests.
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Motivation

I found if I have file path like xx/yy/zz/index.md or xx/yy/zz/readme.md without a 'tittle' frontmatter or '#1' title in the document, then the title of this file shown in the sidebar would be index or readme, which is not desired, since the directory name zz may be more suitable.

Test Plan

Test links

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Related issues/PRs

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[MarkShawn2020]

Note we already have a convention to detect "category index documents", see

Good job！

We also want to cover such subtle behaviors with unit tests

Sorry to admit that I have few ideas on how to write unit tests for directory strucurues.

Also note that with autogeneated sidebars, folders might have a number prefix for ordering reasons, and you may want to strip that number in the infered title: /xx/yy/03-zz/index.md should infer to zz and not 03-zz? (see impl of getSlug() for inspiration)

Yep!