Update index.rst (#5233)

Author: jermainewang

docs/source/notebooks/sparse/index.rst

@@ -1,7 +1,7 @@
 🆕 Tutorials: dgl.sparse
-========================
+=========================
 
-TODO(minjie): intro for the new library.
+The tutorial set cover the basic usage of DGL's sparse matrix class and operators. You can begin with "Quickstart" and "Building a Graph Convolutional Network Using Sparse Matrices". The rest of the tutorials demonstrate the usage by end-to-end examples. All the tutorials are written in Jupyter Notebook and can be played on Google Colab.
 
 .. toctree::
   :maxdepth: 3