feat: add custom maths operator section

undergradmath.typ

@@ -252,6 +252,17 @@ Just type them!
   [$lg$], [`lg`], [$gcd$], [`gcd`], [$lim$], [`lim`],
 ))
 
+#let cosec = math.op("cosec")
+
+If an operator you wish to use does not exist, you can create one using `math.op`. For example, to create the $cosec$ operator:
+
+```
+#let cosec = math.op("cosec")
+$ cosec x = 1/(sin x) $
+```
+
+$ cosec x = 1/(sin x) $
+
 = Other symbols
 #align(center, table(
   columns: 6,
@@ -535,21 +546,14 @@ For permutations use $n^(underline(r))$ from `n^(underline(r))` (some authors us
     E(X) = mu_X = sum(x_i - P(x_i))
     ```
   ],
+  [$ 1 / sqrt(2 sigma^2 pi) e^(- (x - mu)^2 / (2 sigma^2)) $],
+  [
+    ```
+    1 / sqrt(2 sigma^2 pi) e^(- (x - mu)^2 / (2 sigma^2))
+    ```
+  ]
 ))
 
-The probability density of the normal distribution
-
-$ 1 / sqrt(2 sigma^2 pi) e^(- (x - mu)^2 / (2 sigma^2)) $
-
-comes from this.
-
-#align(center, [
-  ```
-  1 / sqrt(2 sigma^2 pi)
-    e^(- (x - mu)^2 / (2 sigma^2))
-  ```
-])
-
 = For more
 See also the Typst Documentation at #link("https://typst.app/docs").