DOC: Add index of tutorials to user guide.

doc/source/user_guide.txt

@@ -2,8 +2,8 @@ User Guide
 ==========
 
 .. toctree::
-   :maxdepth: 1
+   :maxdepth: 2
 
    user_guide/data_types
-   user_guide/segmentation
    user_guide/plugins
+   user_guide/tutorials

doc/source/user_guide/tutorial_segmentation.txt

@@ -114,9 +114,9 @@ use the Sobel operator for computing the amplitude of the gradient::
 From the 3-D surface plot shown below, we see that high barriers effectively
 separate the coins from the background.
 
-.. image:: elevation_map.jpg
+.. image:: data/elevation_map.jpg
     :align: center
- 
+
 Let us now compute the watershed transform::
 
     >>> from skimage.morphology import watershed

doc/source/user_guide/tutorials.txt

@@ -0,0 +1,7 @@
+Tutorials
+=========
+
+.. toctree::
+   :maxdepth: 1
+
+   tutorial_segmentation