Add progressive links between all worksheets

Author: bennuttall

analogue.md

@@ -187,3 +187,4 @@ while True:
 - Use potentiometers to control the visual settings of a Camera Module in real time
 - Find more analogue sensors that will work with the ADC
 - Use potentiometers to control a player in a [PyGame Zero](http://pygame-zero.readthedocs.io) game, or in [Minecraft](https://www.raspberrypi.org/learning/getting-started-with-minecraft-pi/)
+- Continue to the next worksheet on using [Motors](motors.md)

buzzer.md

@@ -44,4 +44,5 @@ Set up the circuit as shown below:
     ```
 
 ## What Next?
-- Have a go at building a [traffic light](trafficlights.md) system using gpiozero.
+
+- Continue to the next worksheet on building a [traffic lights](trafficlights.md) system using GPIO Zero.

distance.md

@@ -124,3 +124,4 @@ Now you've learned to use an ultrasonic distance sensor, you could:
 - Build a proximity sensor alert using a buzzer or a sound file
 - Build a Pi camera photo booth which is activated when a person is close enough to the camera
 - Build a robot with a distance sensor to stop it bumping into other objects
+- Continue to the next worksheet on [analogue inputs](analogue.md)

ldr.md

@@ -66,4 +66,6 @@ Use the following code to set up the light sensor:
 Run this code, then cover the LDR with your hand and watch the value change. Try shining a strong light onto the LDR.
 
 ## What Next?
+
 - You could have a go at using your new knowledge of LDRs to build a [laser-tripwire](https://www.raspberrypi.org/learning/laser-tripwire/).
+- Continue to the next worksheet on using a [Passive Infra-red Sensor](pir.md)

motors.md

@@ -305,7 +305,7 @@ If you had a robot with two wheels you would want to control the two motors toge
 
 ## What next?
 
-Now you've learned how motors work, why not try:
+Now you've learned how motors work, you can try:
 
-- Making your own robot with our [Build a robot](http://raspberrypi.org/learning/robo-butler/) resource?
-- Making a [spinning flower wheel](http://raspberrypi.org/learning/spinning-flower-wheel/)?
+- Making your own robot with our [Build a robot](https://www.raspberrypi.org/learning/robo-butler/) resource
+- Making a [spinning flower wheel](https://www.raspberrypi.org/learning/spinning-flower-wheel/)

pir.md

@@ -31,4 +31,6 @@ while True:
 ```
 
 ## What Next?
+
 - Now you know how to use a PIR, why not have a go at building a [Parent Detector](https://www.raspberrypi.org/learning/parent-detector)
+- Continue to the next worksheet on using an [Ultrasonic Distance Sensor](distance.md)

trafficlights.md

@@ -282,3 +282,4 @@ As well as controlling the whole set of lights together, you can also control ea
 
 - Try adding a second button for the other side of the road. You'll probably need to use GPIO Zero `button.when_pressed` rather than `wait_for_press`, which can only be used for one button at a time.
 - Refer to the documentation at [gpiozero.readthedocs.org](https://gpiozero.readthedocs.org/) for more information on what can be done with GPIO Zero.
+- Continue to the next worksheet on using a [Light Dependent Resitor](ldr.md)

worksheet.md

@@ -202,3 +202,4 @@ There are lots of other things you can control or monitor with your Raspberry Pi
 - [Using a PIR Sensor](pir.md)  
 - [Using an ultrasonic distance sensor](distance.md)
 - [Analogue inputs](analogue.md)
+- [Using motors](motors.md)