Add starting point examples (#731)

* Also put full description in shortDescription to avoid loading 2 different descriptions in the app

Author: ClementPasteau

__tests__/post-build/exampleShortHeaders.json.spec.js

@@ -33,7 +33,7 @@ describe('exampleShortHeaders.json post build checks', () => {
         name: 'Platformer',
         authorIds: ['R0F5QGNCzgOY5w2cxGeKJOq2UaD2'],
         shortDescription:
-          'An example of a basic platformer (Mario-like) game. Jump around and collect as many coins as you can!',
+          'An example of a basic platformer (Mario-like) game. Jump around and collect as many coins as you can!\n\nCheck out [the YouTube video](https://www.youtube.com/watch?v=eU0kkLSdw0Y&list=PL3YlZTdKiS898Wio0tvKjQM0x3zo4V0Mb) explaining how this example was made.',
         license: 'MIT',
         previewImageUrls: [
           'https://resources.gdevelop-app.com/examples/platformer/preview.png',

examples/starting-physics/README.md

@@ -0,0 +1 @@
+Starting point for a physics game.

examples/starting-physics/TAGS.md

@@ -0,0 +1 @@
+starting-point