tpl/math: Add math.Rand template function

Closes #11833

docs/content/en/functions/math/Rand.md

@@ -0,0 +1,44 @@
+---
+title: math.Rand
+description: Returns a pseudo-random number in the half-open interval [0.0, 1.0).
+categories: []
+keywords: []
+action:
+  aliases: []
+  related: []
+  returnType: float64
+  signatures: [math.Rand]
+---
+
+The `math.Rand` function returns a pseudo-random number in the [half-open interval] [0.0, 1.0).
+
+```go-html-template
+{{ math.Rand }} → 0.6312770459590062
+```
+
+To generate a random integer in the [closed interval] [0, 5]:
+
+```go-html-template
+{{ math.Rand | mul 6 | math.Floor }}
+```
+
+To generate a random integer in the closed interval [1, 6]:
+
+```go-html-template
+{{ math.Rand | mul 6 | math.Ceil }}
+```
+
+To generate a random float, with one digit after the decimal point, in the closed interval [0, 4.9]:
+
+```go-html-template
+{{ div (math.Rand | mul 50 | math.Floor) 10 }}
+```
+
+To generate a random float, with one digit after the decimal point, in the closed interval [0.1, 5.0]:
+
+```go-html-template
+{{ div (math.Rand | mul 50 | math.Ceil) 10 }}
+```
+
+[closed interval]: /getting-started/glossary/#interval
+[half-open interval]: /getting-started/glossary/#interval