fix: support use same name file as sidebar index when no _meta.json

[Timeless0911]

Summary

1. If there exists a file with the same name of the directory folder, we use it for sidebar index
2. if 1, the sidebar label will be the H1 title of the same name file

Related Issue

Checklist

• [x] Tests updated (or not required).
• [ ] Documentation updated (or not required).

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