Use raw strings for math equations (#881)

This fixes “Math input error” which can be seen in the documentation:
https://sphinx-book-theme.readthedocs.io/en/stable/reference/notebooks.html#math

Author: mitya57

docs/content/notebooks.md

@@ -115,7 +115,7 @@ print("this works for code cells too, if you add a `margin` tag to them")
 ```{code-cell} ipython3
 # You can also include enriched outputs like Math
 from IPython.display import Math
-Math("\sum_{i=0}^n i^2 = \frac{(n^2+n)(2n+1)}{6}")
+Math(r"\sum_{i=0}^n i^2 = \frac{(n^2+n)(2n+1)}{6}")
 ```
 
 ## Removing content before publishing

docs/reference/notebooks.md

@@ -123,7 +123,7 @@ plt.show()
 ```{code-cell} ipython3
 # You can also include enriched outputs like Math
 from IPython.display import Math
-Math("\sum_{i=0}^n i^2 = \frac{(n^2+n)(2n+1)}{6}")
+Math(r"\sum_{i=0}^n i^2 = \frac{(n^2+n)(2n+1)}{6}")
 ```
 
 ### Pandas DataFrames