[Doc] sparse lib tutorial and colab links (#5181)

* Created using Colaboratory

* nbsphinx & nblink

* Created using Colaboratory

* update notebook

* minor update

docs/source/conf.py

@@ -48,6 +48,8 @@ extensions = [
     'sphinx.ext.graphviz',
     'sphinx_gallery.gen_gallery',
     'sphinx_copybutton',
+    'nbsphinx',
+    'nbsphinx_link',
 ]
 
 # Add any paths that contain templates here, relative to this directory.

docs/source/index.rst

@@ -25,6 +25,7 @@ Welcome to Deep Graph Library Tutorials and Documentation
    guide/index
    guide_cn/index
    guide_ko/index
+   notebooks/sparse/index
    tutorials/large/index
    tutorials/cpu/index
    tutorials/multi/index
@@ -51,10 +52,10 @@ Welcome to Deep Graph Library Tutorials and Documentation
    api/python/dgl.ops
    api/python/dgl.optim
    api/python/dgl.sampling
+   api/python/dgl.sparse_v0
    api/python/dgl.multiprocessing
    api/python/transforms
    api/python/udf
-   api/python/dgl.sparse_v0
 
 .. toctree::
    :maxdepth: 1

docs/source/notebooks/sparse/index.rst

+🆕 Tutorials: dgl.sparse
+========================
+
+TODO(minjie): intro for the new library.
+
+.. toctree::
+  :maxdepth: 2
+  :titlesonly:
+
+  quickstart.nblink

docs/source/notebooks/sparse/quickstart.nblink

+{
+    "path": "../../../../notebooks/sparse/quickstart.ipynb"
+}