_index.md with no title inherits name from folder name

[Visual-Dawg]

Feature Description

Currently when an _index.md within a folder has no frontmatter title set, it will not have a title. So the sidebar entry is blank and it shows up as _index in the search results.

Problem/Solution.

While adding a title manually is quite simple, inheriting it in this case from the parent folder makes migrating from other themes who have this feature, like Geekdocs, easier.

Alternatives Considered
Manually setting the title, which is also fine.