docs: update deepmd-gnn URL (#4482)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Documentation**
- Updated guidelines for creating and integrating new models in the
DeePMD-kit framework.
- Added new sections on descriptors, fitting networks, and model
requirements.
	- Enhanced unit testing section with instructions for regression tests.
- Updated URL for the DeePMD-GNN plugin to reflect new repository
location.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Jinzhe Zeng <[email protected]>

doc/development/create-a-model-pt.md

@@ -180,7 +180,7 @@ The arguments here should be consistent with the class arguments of your new com
 ## Package new codes
 
 You may package new codes into a new Python package if you don't want to contribute it to the main DeePMD-kit repository.
-A good example is [DeePMD-GNN](https://github.com/njzjz/deepmd-gnn).
+A good example is [DeePMD-GNN](https://gitlab.com/RutgersLBSR/deepmd-gnn).
 It's crucial to add your new component to `project.entry-points."deepmd.pt"` in `pyproject.toml`:
 
 ```toml

doc/third-party/out-of-deepmd-kit.md

@@ -6,7 +6,7 @@ The codes of the following interfaces are not a part of the DeePMD-kit package a
 
 ### External GNN models (MACE/NequIP)
 
-[DeePMD-GNN](https://github.com/njzjz/deepmd-gnn) is DeePMD-kit plugin for various graph neural network (GNN) models.
+[DeePMD-GNN](https://gitlab.com/RutgersLBSR/deepmd-gnn) is DeePMD-kit plugin for various graph neural network (GNN) models.
 It has interfaced with [MACE](https://github.com/ACEsuit/mace) (PyTorch version) and [NequIP](https://github.com/mir-group/nequip) (PyTorch version).
 It is also the first example to the DeePMD-kit [plugin mechanism](../development/create-a-model-pt.md#package-new-codes).