docs: fix broken links to source of graph classification datasets (#8946

)

**Update Broken Dataset Links in Documentation**

This PR addresses broken links in the documentation that pointed to the
common benchmark datasets. The links were updated to point to the
correct URL.

Changes were made in the following files:

1. `benchmark/kernel/README.md`
2. `docs/source/get_started/introduction.rst`

The specific changes are as follows:

In `benchmark/kernel/README.md`:

```diff
- Evaluation script for various methods on [common benchmark datasets](http://graphkernels.cs.tu-dortmund.de) via 10-fold cross validation, where a training fold is randomly sampled to serve as a validation set.
+ Evaluation script for various methods on [common benchmark datasets](https://chrsmrrs.github.io/datasets/) via 10-fold cross validation, where a training fold is randomly sampled to serve as a validation set.Update Broken Dataset Links in Documentation
```

In `docs/source/get_started/introduction.rst`:

```diff
- :pyg:`PyG` contains a large number of common benchmark datasets, *e.g.*, all Planetoid datasets (Cora, Citeseer, Pubmed), all graph classification datasets from `http://graphkernels.cs.tu-dortmund.de <http://graphkernels.cs.tu-dortmund.de/>`_ and their `cleaned versions <https://github.com/nd7141/graph_datasets>`_, the QM7 and QM9 dataset, and a handful of 3D mesh/point cloud datasets like FAUST, ModelNet10/40 and ShapeNet.
+ :pyg:`PyG` contains a large number of common benchmark datasets, *e.g.*, all Planetoid datasets (Cora, Citeseer, Pubmed), all graph classification datasets from `TUDatasets https://chrsmrrs.github.io/datasets/`_ and their `cleaned versions <https://github.com/nd7141/graph_datasets>`_, the QM7 and QM9 dataset, and a handful of 3D mesh/point cloud datasets like FAUST, ModelNet10/40 and ShapeNet.
```

Please review these changes and merge the PR if everything is in order
or let me know if there are any issues or further changes needed.

---------

Co-authored-by: Ravi Kalia <rkalia.consultant@kriyatx.com>
Co-authored-by: Matthias Fey <matthias.fey@tu-dortmund.de>

benchmark/kernel/README.md

@@ -1,6 +1,6 @@
 # Graph Classification
 
-Evaluation script for various methods on [common benchmark datasets](http://graphkernels.cs.tu-dortmund.de) via 10-fold cross validation, where a training fold is randomly sampled to serve as a validation set.
+Evaluation script for various methods on [common benchmark datasets](https://chrsmrrs.github.io/datasets/) via 10-fold cross validation, where a training fold is randomly sampled to serve as a validation set.
 Hyperparameter selection is performed for the number of hidden units and the number of layers with respect to the validation set:
 
 - **[GCN](https://github.com/pyg-team/pytorch_geometric/blob/master/benchmark/kernel/gcn.py)**

docs/source/get_started/introduction.rst

@@ -129,7 +129,7 @@ You can find a complete list of all methods at :class:`torch_geometric.data.Data
 Common Benchmark Datasets
 -------------------------
 
-:pyg:`PyG` contains a large number of common benchmark datasets, *e.g.*, all Planetoid datasets (Cora, Citeseer, Pubmed), all graph classification datasets from `http://graphkernels.cs.tu-dortmund.de <http://graphkernels.cs.tu-dortmund.de/>`_ and their `cleaned versions <https://github.com/nd7141/graph_datasets>`_, the QM7 and QM9 dataset, and a handful of 3D mesh/point cloud datasets like FAUST, ModelNet10/40 and ShapeNet.
+:pyg:`PyG` contains a large number of common benchmark datasets, *e.g.*, all Planetoid datasets (Cora, Citeseer, Pubmed), all graph classification datasets from `TUDatasets <https://chrsmrrs.github.io/datasets/>`_ and their `cleaned versions <https://github.com/nd7141/graph_datasets>`_, the QM7 and QM9 dataset, and a handful of 3D mesh/point cloud datasets like FAUST, ModelNet10/40 and ShapeNet.
 
 Initializing a dataset is straightforward.
 An initialization of a dataset will automatically download its raw files and process them to the previously described :class:`~torch_geometric.data.Data` format.