Add doc for the development of core and experimental sections (#13966)

### **Description**

Hi, I just started learning the source code of `langchain` and hope to
contribute code. However, according to the instructions in the
[CONTRIBUTING.md](https://github.com/langchain-ai/langchain/blob/master/.github/CONTRIBUTING.md)
document, I could not run the test command `make test` to run normally.
I found that many modules did not exist after [splitting
`langchain_core`](https://github.com/langchain-ai/langchain/discussions/13823),
so I updated the document.

### **Twitter handle** 

lin_bob57617