Point to README.md#from-source instead of duplicate instructions in CONTRIBUTING.md#developing-pytorc (#91850)
**Idea:** [README.md#from-source](https://github.com/pytorch/pytorch/blob/master/README.md#from-source) should be the place that describes how I as a developer builds from source.
Currently, `CONTRIBUTING.md` suggests an incomplete set of install instructions that predates those in `README.md`.
This PR tries to simplify and remove a dead end from the developer onboarding funnel by pointing to [README.md#from-source](https://github.com/pytorch/pytorch/blob/master/README.md#from-source).
### Details
Without touching this codebase for years I tried to build repo for local development and run unit tests. I tried to capitalise on the confusion by documenting it:
1. I go to [README.md#from-source](https://github.com/pytorch/pytorch/blob/master/README.md#from-source)
2. Since it doesn't suggest how I run unit test I follow [README.md#releases-and-contributing to ](https://github.com/pytorch/pytorch/blob/master/README.md#releases-and-contributing) to [CONTRIBUTING.md#developing-pytorch](https://github.com/pytorch/pytorch/blob/master/CONTRIBUTING.md#developing-pytorch) which is written as if it's _the_ set up dev env instruction:
https://github.com/pytorch/pytorch/blob/73e5379fab05c40ff6f42500309cdc17ee57548c/CONTRIBUTING.md?plain=1#L88-L90
But this section gives competing and incomplete install instructions that does not work for me. Ex, it doesn't mention `ninja` or `pyaml` required for `python setup.py develop`.
5. Going back to the original [README.md#from-source](https://github.com/pytorch/pytorch/blob/master/README.md#from-source) setup instructions that (mostly) worked.
https://github.com/pytorch/pytorch/blob/73e5379fab05c40ff6f42500309cdc17ee57548c/README.md?plain=1#L187
#### TODO
- [x] verify that it does not break any link to other documentation
[skip ci]
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91850
Approved by: https://github.com/ZainRizvi, https://github.com/seemethere
