feat(nested collections): allow non-index files

[fgnass]

This PR is based on #6498 and resolves all outstanding merge conflicts. It recreates the necessary test snapshots and ensures the feature aligns with the latest codebase.

Summary

This commit fixes #4972 to allow nested folders with additional content beyond an index file.

Side effect: To keep the feature simple, this will now show index files as pages within a folder in NetlifyCMS. This enables creating additional files alongside the given index, but is a change in behavior from the current implementation.

Additionally, this PR:

• Resolves merge conflicts that prevented feat(nested collections): allow additional content in flatter structure #6498 from being merged.
• Recreates the test snapshot for NestedCollection.spec.js to align with the updated codebase.

Test Plan

1. Run the test suite:

   npm test
   

2. Verify that all tests, including the recreated snapshot for NestedCollection.spec.js, pass successfully.
3. Manually test the behavior of nested folders with additional content to ensure it functions as expected.

Checklist

• I have read the contribution guidelines.
• Tests have been updated to reflect changes.
• Changes have been manually verified.

A picture of a cute animal (not mandatory but encouraged)

[demshy]

Amazing work once again @fgnass .

I discussed and tested this with @martinjagodic against some of our projects that use this. I don’t think the breaking change dramatically changes the behaviour for current implementations, especially since the whole feature is still marked as Beta.

I suggest release this and discuss some kind of a temporary legacy mode in case of some reasonable concerns arise from the community.

We did encounter a semi-related issue though. When getNodeTitle obtains the title it will take the title of the first child, which is the _index file unless in this edge case there is a file name that starts with a number. In this case this will be the title of the section. We will investigate this a bit and open an issue that will address this and maybe even add a possibility to take the dir name as nodeTitle when _index file is not present.