[Feature Request] Source installation docs should pin supported PyTorch versions

### Summary

The current “install from source” documentation does not clearly require a supported PyTorch version.

When installing from source with a normal `pip install`, pip may automatically install or upgrade PyTorch to 2.13.0 (as of 2026/7/11). However, DeePMD-kit currently only supports `torch.compile` with PyTorch 2.11.x or 2.12.x, which later causes runtime errors such as:

```text
RuntimeError: deepmd `torch.compile` support requires PyTorch 2.11.x or 2.12.x; found torch 2.13.0+cu126.
```

Please update the source installation documentation to either:

* explicitly pin a supported version, for example `torch==2.11.0` or `torch>=2.11,<2.13`; or
* add a prominent warning that users must install a supported PyTorch version before installing DeePMD-kit from source.

It would also be helpful if the package metadata imposed the same upper bound, so pip cannot silently install an incompatible PyTorch version.

### Detailed Description

As above

### Further Information, Files, and Links

_No response_