Add a custom render phase example

[IceSentry]

Objective

• It's currently very hard for beginners and advanced users to get a full understanding of a complete render phase

Solution

• Implement a full custom render phase

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Showcase

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• If this PR adds a new feature or public API, consider adding a brief pseudo-code snippet of it in action
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  • If you want, you could even include a before/after comparison!
• If the Migration Guide adequately covers the changes, you can delete this section

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[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.