Showing DAGs in folder like structure

### Description

The DAGs main page looks messy when we've too many dags. What if we can create a soft foldering around it? I mean a UI-only folder structure to group DAGs. There could be 2 possible ways to group (or maybe more). The first way is grouping via subpaths under DAGS_FOLDER where the actual dag file is present. The second way is via tags. There could be a toggle button at the top to get the flat list structure as we've at present.

@bradarndt @ryanahamilton what do you guys think?

### Use case/motivation

This feature would help keep the DAGs main page neat and clean. It would also motivate the users to arrange the dags as per their functionalities.

### Related issues

_No response_

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

Start by reviewing the DAGs main page and the existing flat-list behavior. Compare grouping by DAG file subpaths with grouping by tags, and define how the toggle should work; done means the page can show an organized grouped view while retaining the current flat list.

Written by the indexing model from the issue text.