Fixes#155028
This pull request updates the documentation by transitioning from .rst to .md format. It introduces new Markdown files for the documentation of named_tensor, nested, nn.attention.bias, nn.attention.experimental, and nn.attention.flex_attention
Pull Request resolved: https://github.com/pytorch/pytorch/pull/155696
Approved by: https://github.com/svekars
Co-authored-by: Svetlana Karslioglu <svekars@meta.com>