Fix example URLs in TheThingsNetwork documentation

The ABP and OTAA examples links were no longer correct, also replaced
all absolute Github URLs with relative ones so they work as expected in
forked or offline repositories.

Author: cimm

CONTRIBUTING.md

@@ -3,7 +3,7 @@
 We'd ❤️ to welcome your contribution. Please make sure your changes include:
 
 1.  Examples: [examples](examples)
-2.  Syntax highlighting: [keywords.txt](../keywords.txt)
+2.  Syntax highlighting: [keywords.txt](keywords.txt)
 3.  Documentation: [docs](docs)
 4.  Tests: [test](test)
 5.  Verification of the examples and tests: [.travis.yml](.travis.yml)

docs/TheThingsNetwork.md

@@ -64,7 +64,7 @@ RX Delay 1: 1000
 RX Delay 2: 2000
 ```
 
-See the [DeviceInfo](https://github.com/TheThingsNetwork/arduino-device-lib/blob/master/examples/DeviceInfo/DeviceInfo.ino) example.
+See the [DeviceInfo](../examples/DeviceInfo/DeviceInfo.ino) example.
 
 ## Method: `onMessage`
 
@@ -78,7 +78,7 @@ void onMessage(void (*cb)(const byte* payload, size_t length, port_t port));
 - `size_t length`: Number of bytes.
 - `port_t port`: The port addressed.
 
-See the [Receive](https://github.com/TheThingsNetwork/arduino-device-lib/blob/master/examples/Receive/Receive.ino) example.
+See the [Receive](../examples/Receive/Receive.ino) example.
 
 ## Method: `join`
 
@@ -115,7 +115,7 @@ Returns `true` or `false` depending on whether the activation was successful.
 
 Call the method with no arguments if the device's LoRa module comes with pre-flashed values.
 
-See the [ABP](https://github.com/TheThingsNetwork/arduino-device-lib/blob/master/examples/ABP/ABP.ino) example.
+See the [SendABP](../examples/SendABP/SendABP.ino) example.
 
 ## Method: `sendBytes`
 
@@ -138,7 +138,7 @@ Returns a success or error code and logs the related error message:
 * `TTN_SUCCESSFUL_RECEIVE`: Successful transmission. Received \<N> bytes
 * `TTN_ERROR_UNEXPECTED_RESPONSE`: Unexpected response: \<Response>
 
-See the [Send](https://github.com/TheThingsNetwork/arduino-device-lib/blob/master/examples/Send/Send.ino) example.
+See the [SendOTAA](../examples/SendOTAA/SendOTAA.ino) example.
 
 ## Method: `poll`
 
@@ -153,7 +153,7 @@ int8_t poll(port_t port = 1, bool confirm = false);
 
 Returns the result of `sendBytes()`.
 
-See the [Receive](https://github.com/TheThingsNetwork/arduino-device-lib/blob/master/examples/Receive/Receive.ino) example.
+See the [Receive](../examples/Receive/Receive.ino) example.
 
 ## Method: `provision`
 
@@ -217,4 +217,3 @@ Returns the voltage in millivolt (mV) measured by the RN2xxx LoRa module. It's f
 ```c
 uint16_t getVDD();
 ```
-