new example showcase tool

[mockersf]

Objective

• Replace the example_showcase.sh script
• Helper tool to prepare the example page on the website

Solution

• Have a command to run all the examples: cargo run -p example-showcase -- run
• Have a command to take screenshots of all examples: cargo run -p example-showcase -- run --screenshot
• Have a command to build the markdown files for the website: cargo run -p example-showcase -- build-website-list --content-folder content
• Have a command to build all the examples in wasm/WebGPU: cargo run -p example-showcase -- build-web-gpu-examples --content-folder webgpus (with --website-hacks to enable the hacks for the Bevy website: canvas id, resizing and loading bar)

This is the first step to an improved example page (all examples marked as wasm, uses the card layout, has screenshots, reuse name, category and description from the metadata). As one of the goal is to have a page with WebGPU examples before the official release, this is not touching the example page for now but targeting a new one.