Functional interrupt fix (#8175)

* Added link to external examples to the doc

* Fixed FunctionalInterrupt.ino + added introduction comment and few outputs

libraries/ESP32/examples/GPIO/FunctionalInterrupt/FunctionalInterrupt.ino

+/*
+ * This example demonstrates usage of interrupt by detecting a button press.
+ *
+ * Setup: Connect first button between pin defined in BUTTON1 and GND
+ *        Similarly connect second button between pin defined in BUTTON2 and GND.
+ *        If you do not have a button simply connect a wire to those buttons
+ *        - touching GND pin with other end of the wire will behave same as pressing the connected button.
+ *        Wen using the bare wire be careful not to touch any other pin by accident.
+ *
+ * Note: There is no de-bounce implemented and the physical connection will normally
+ *       trigger many more button presses than actually happened.
+ *       This is completely normal and is not to be considered a fault.
+ */
+
+
 #include <Arduino.h>
 #include <FunctionalInterrupt.h>
 
 #define BUTTON1 16
 #define BUTTON2 17
 
-class Button
-{
+class Button{
 public:
   Button(uint8_t reqPin) : PIN(reqPin){
     pinMode(PIN, INPUT_PULLUP);
-    attachInterrupt(PIN, std::bind(&Button::isr,this), FALLING);
   };
-  ~Button() {
+
+  void begin(){
+    attachInterrupt(PIN, std::bind(&Button::isr,this), FALLING);
+    Serial.printf("Started button interrupt on pin %d\n", PIN);
+  }
+
+  ~Button(){
     detachInterrupt(PIN);
   }
 
-  void ARDUINO_ISR_ATTR isr() {
+  void ARDUINO_ISR_ATTR isr(){
     numberKeyPresses += 1;
     pressed = true;
   }
 
-  void checkPressed() {
+  void checkPressed(){
     if (pressed) {
       Serial.printf("Button on pin %u has been pressed %u times\n", PIN, numberKeyPresses);
       pressed = false;
@@ -36,9 +55,13 @@ private:
 Button button1(BUTTON1);
 Button button2(BUTTON2);
 
-
 void setup() {
     Serial.begin(115200);
+    while(!Serial) delay(10);
+    Serial.println("Starting Functional Interrupt example.");
+    button1.begin();
+    button2.begin();
+    Serial.println("Setup done.");
 }
 
 void loop() {