Add description to starters and possibility to add a preview image

Don't show in changelog

Author: AlexandreSi

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

@@ -31,9 +31,10 @@ describe('filters.json post build checks', () => {
       name: 'Platformer',
       authorIds: [],
       shortDescription:
-        'An example of a basic platformer (Mario-like) game made with GDevelop.',
+        'An example of a basic platformer (Mario-like) game. Jump around and collect as many coins as you can!',
       license: 'MIT',
       previewImageUrls: [
+        'https://resources.gdevelop-app.com/examples/platformer/preview.png',
         'https://resources.gdevelop-app.com/examples/platformer/thumbnail.png',
       ],
       tags: [

examples/downhill-bike-physics-demo/README.md

@@ -1 +1,3 @@
-A fun demo, using the Physics Engine to build an advanced simulation of a bike, with a person pedaling on it. Try to cross the finish line without falling and as fast as possible!
+Try to cross the finish line as fast as possible without falling over! And remember to pedal safe!
+
+Starting with this template, use the Physics Engine to build an advanced simulation of a bike with a person pedaling on it! You will discover how to join objects, apply basic physics behaviors to your scene and change your camera settings to adapt to what's happening during the game.

examples/downhill-bike-physics-demo/preview.png

@@ -1 +1,3 @@
 A demo showing how to enhance the "game feel" of your project: VFX, shot trail, ambient sounds and SFX, screenshake, wobble...
+
+Have you ever felt like you were actually inside a video game when playing it? Discover GDevelop's capabilities and immerse future players in your next game!

examples/game-feel-demo/README.md

@@ -1 +1,3 @@
 A hyper casual endless game where you have to collect shapes and avoid bombs, with a progressively increasing difficulty.
+
+Start from this example to create your own catch game! You will learn about collisions, particle emitters, and how to set up a menu.

examples/game-feel-demo/preview.png

@@ -1 +1,3 @@
-Example of making an isometric 2D game.
+An example of a 2D isometric game. If you ever dreamed of creating you own version of Civilization or Warcraft, this example is for you!
+
+Discover what you can accomplish within an isometric world using smooth animations. Add as many blocks as you want to create whatever you can imagine!

examples/geometry-monster/README.md

@@ -1 +1,3 @@
 Find all matching pairs of cards in this relaxing game. Use tweens to create smooth, natural animations with a few events.
+
+Get going with your first GDevelop game: matching pairs games can seem to be easy to create, but you will learn a lot using tweens, timers and variables (for the score)!