Improve examples

[Shatur]

We need to make the following changes to our examples to better teach users:

• Write a proper character controller example instead of the current one. See https://bevy.org/examples/movement/physics-in-fixed-timestep/ for more details. To put it short, it should run in FixedUpdate and accumulate input. Check how bevy_ahoy does it.
• Add pull_style_api that mirrors basic_action_management, but uses pull-style API.
• Rework local_multiplayer showcases a character controller that runs in Update, which isn't nice (see the link above). Let's change it into a snake game with 2 snakes (to showcase local multiplayer). In this kind of game we can mention that it's ok to use Update. It also sounds fun 😅(offtopic, but I also recently discovered a snake MMO via SSH). Adjust local_multiplayer as well.

Each change could be done in a separate PR. Contributions are welcome. I think it's a good opportunity for new contributors to play with the crate and Bevy in general 🙂

[jayhwkns]

How do we feel about that third bullet point now that the local_multiplayer example was changed to something else? I just opened a PR setting up the fixed update for the character controller, so I can pretty easily go back and change that too (unless we really want to keep that one simple).

[Shatur]

@jayhwkns I think it's worth keeping what you did and just adjust it to use fixed timestamp.