Added notes for Laplace Operator

Chapters/15-Appendix-A/INDEX.md

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+# Appendix A
+
+## Laplace Operator[^khan-1]
+
+It is defined as $\nabla \cdot \nabla f \in \R$ and is equivalent to the
+**divergence of the function**. Technically speaking it gives us the
+**magnitude of a local maximum or minimum**.
+
+Positive values mean that we are around a local maximum and vice-versa. The
+higher the magnitude, the higher (or lower) is the local maximum (or minimum).
+
+Another way to see this is as the divergence of the function that tells us whether
+that is a point of attraction or divergence.
+
+It can also be used to compute the net flow of particles in that region of space
+
+> [!CAUTION]
+> This is not a **discrete laplace operator**, which is instead a **matrix** here,
+> as there are many other formulations.
+
+[^khan-1]: [Khan Academy | Laplace Intuition | 9th November 2025](https://www.youtube.com/watch?v=EW08rD-GFh0)
+