Fix bugs in nested collection

[pranaysahith]

Summary

Fix bugs in nested collection when subfolders is configured to false.
See issues #7606 #7651

Nested Collection as a regular directory file structure with subfolders false

• Make index file optional for nested collections
• Sidebar names depend on index files for nested collections, instead now it uses folder names
• Able to create New files in specific folders when meta.path is configured
• New file names to be derived from title field in the frontmatter
• Able to move files between folders without moving folder or sibiling files when meta.path is configured

Test plan

• Existing Unit tests updated with subfolders=true config to test backward compatibility.
• New Unit tests added to test new functionality.

Checklist

Please add a x inside each checkbox:

• [ x] I have read the contribution guidelines.
• [ x] All tests passing

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