Document the examples

Nobody has claimed this yet.

The Pygame Zero codebase contains [a number of example games](https://github.com/lordmauve/pgzero/tree/master/examples).

Our documentation should explain how to install and run these examples, from a point of view of a user who has Pygame Zero installed but not the example games. For example, it might recommend downloading the repository as a zip.

Additionally, for each game, we might include a screenshot and a brief explanation of what the user could learn by reading or modifying the code of the game.

Start by reviewing the games in the repository's examples/ directory and the existing documentation structure. Document how a user with Pygame Zero installed can obtain and run the examples, then add a screenshot and brief learning-focused explanation for each game; completion means all examples are covered and the instructions are usable without already having the repository.

Written by the indexing model from the issue text.