Structure https://github.com/linkedin/feathr/tree/main/docs/dev_guide [FR]

### Willingness to contribute

Yes. I can contribute a fix for this bug independently.

### Feature Request Proposal

structure the https://github.com/linkedin/feathr/tree/main/docs/dev_guide. Right now, it's all flat so it's pretty hard to navigate. Either create a homepage for navigation or put into reasonable nested folders.

### Motivation

> #### What is the use case for this feature?
better doc readability

### Details

_No response_

### What component(s) does this bug affect?

- [X] `Python Feathr Client`: This is the client users use to interact with most of our API. Mostly written in Python.
- [X] `Computation Engine`: The computation engine that execute the actual feature join and generation work. Mostly in Scala and Spark.
- [X] `Feature Registry API Layer`: The storage layer supports SQL, Purview(Atlas). The API layer is in Python(FAST API)
- [X] `Feature Registry Web UI layer`: The Web UI for feature registry. Written in React with a few UI frameworks.