Docs: add CSPNext to pytorch architectures, minor formatting (#3413)

* Fix incorrect docstring

* Document CSPNeXt and tidy architecture headings

Update the PyTorch architectures docs to use consistent `###` subsection headings for each model family, improving readability and navigation. Add a new CSPNeXt section describing its role as a standalone backbone and as RTMPose’s backbone, including available variants (`cspnext_s`, `cspnext_m`, `cspnext_x`).

* Update cspnext.py