Add FP framing to functions and void/never exercises

- Functions README: Introduce "transformations" and pure functions concept
- Void/Never README: Reframe void as exception, emphasize side effects

Author: kentcdodds

exercises/04.functions/README.mdx

@@ -9,6 +9,13 @@ function add(a: number, b: number): number {
 }
 ```
 
+<callout-info>
+	Think of functions as **transformations**: data goes in, data comes out.
+	Functions that always produce the same output for the same input (and don't
+	change anything outside themselves) are called **pure functions**. They're the
+	foundation of reliable, testable code.
+</callout-info>
+
 ## Anatomy of a TypeScript Function
 
 ```ts

exercises/05.void-and-never/README.mdx

@@ -22,6 +22,12 @@ Common side effects:
 - Sending analytics events
 - Writing to a database
 
+<callout-info>
+	Most functions should return a value (be "pure"). Functions with `void` return
+	types are the exception—they exist to cause side effects. When you see `void`,
+	you know the function is doing something beyond computing a result.
+</callout-info>
+
 ## The `never` Type
 
 `never` means "this function never returns." It either: