Demodocs: add math label and equation link

Author: Blendify

demo_docs/source/index.rst

@@ -49,7 +49,7 @@ Here is another:
     \frac{1}{r^2 \sin \theta} \frac{\partial f}{\partial \theta}
     \left( \sin \theta \, \frac{\partial f}{\partial \theta} \right) +
     \frac{1}{r^2 \sin^2\theta} \frac{\partial^2 f}{\partial \phi^2}
-    
+
 You can add a link to equations like the one above :eq:`This is a label` by using ``:eq:``.
 
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