docs: add example programs

- hello.bux: Hello World with PrintLine
- fibonacci.bux: Recursive Fibonacci with while loop
- factorial.bux: Recursive Factorial computation
- structs.bux: Struct creation and field access

All examples compile and run successfully via 'bux run'

examples/fibonacci.bux

@@ -0,0 +1,24 @@
+// Fibonacci - Recursive function with while loop
+extern func Std_Io_PrintLine(s: String);
+extern func Std_Io_PrintInt(n: int);
+
+func Fibonacci(n: int) -> int {
+    if n <= 1 {
+        return n;
+    }
+    return Fibonacci(n - 1) + Fibonacci(n - 2);
+}
+
+func Main() -> int {
+    Std_Io_PrintLine("Fibonacci sequence:");
+    
+    var i: int = 0;
+    while i < 10 {
+        let fib: int = Fibonacci(i);
+        Std_Io_PrintInt(fib);
+        Std_Io_PrintLine("");
+        i = i + 1;
+    }
+    
+    return 0;
+}