Update OSS nested tensor docs to focus on NJT (pytorch#145402)

Updated nested tensor docs to be NJT-centric (instead of NST-centric). They now include:
* High-level description of NST vs. NJT + a recommendation to use NJT
* General NJT construction / usage
* torch.compile() integration w/ dynamic shapes
* Common errors and how to fix them
* Contribution guide
* Data layout / shape information (with diagram)
* Links to more extensive tutorials involving Transformers / SDPA / FlexAttention

Pull Request resolved: pytorch#145402
Approved by: https://github.com/soulitzer

Author: jbschlosser