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/hello.bux

@@ -0,0 +1,7 @@
+// Hello World - Basic Bux program
+extern func Std_Io_PrintLine(s: String);
+
+func Main() -> int {
+    Std_Io_PrintLine("Hello, Bux!");
+    return 0;
+}