docs: improve the docs by provide better examples and describe module publishing (#12308)

examples/compiletime/reflection.v

@@ -0,0 +1,49 @@
+// An example deserializer implementation
+
+struct User {
+	name string
+	age  int
+}
+
+fn main() {
+	data := 'name=Alice\nage=18'
+	user := decode<User>(data)
+	println(user)
+}
+
+fn decode<T>(data string) T {
+	mut result := T{}
+	// compile-time `for` loop
+	// T.fields gives an array of a field metadata type
+	$for field in T.fields {
+		$if field.typ is string {
+			// $(string_expr) produces an identifier
+			result.$(field.name) = get_string(data, field.name)
+		} $else $if field.typ is int {
+			result.$(field.name) = get_int(data, field.name)
+		}
+	}
+	return result
+}
+
+fn get_string(data string, field_name string) string {
+	for line in data.split_into_lines() {
+		key_val := line.split('=')
+		if key_val[0] == field_name {
+			return key_val[1]
+		}
+	}
+	return ''
+}
+
+fn get_int(data string, field string) int {
+	return get_string(data, field).int()
+}
+
+// `decode<User>` generates:
+// fn decode_User(data string) User {
+//     mut result := User{}
+//     result.name = get_string(data, 'name')
+//     result.age = get_int(data, 'age')
+//     return result
+// }