docs: simplify icp new command in quickstart (#266)

## Summary

- Replace the three explicit `--define` flags with `--silent`, which
applies the template's default values without an interactive prompt
- Switch project name from `my-project` to `hello-icp` for a more
intentional tutorial narrative
- Update explanation text to describe `--silent` and introduce
`--define` as the override pattern
- Update the "Prefer Rust?" callout to match the new command pattern

## Sync recommendation

`informed by dfinity/icp-cli — docs/quickstart.md`

Author: marc0olo

docs/getting-started/quickstart.md

@@ -42,15 +42,12 @@ ic-wasm --version
 ## Create a project
 
 ```bash
-icp new my-project --subfolder hello-world \
-  --define backend_type=motoko \
-  --define frontend_type=react \
-  --define network_type=Default && cd my-project
+icp new hello-icp --subfolder hello-world --silent && cd hello-icp
 ```
 
-This creates a fullstack project from the `hello-world` template with a Motoko backend and React frontend. The `--define` flags skip interactive prompts. Without them, `icp new` asks you to choose a template, language, and network type.
+This creates a fullstack project from the [`hello-world` template](https://github.com/dfinity/icp-cli-templates/tree/main/hello-world) with a Motoko backend and React frontend. `--silent` skips the interactive prompt and applies the template's default values. To use different values, add `--define <placeholder>=<value>` (for example, `--define backend_type=rust`). See the [template placeholders](https://github.com/dfinity/icp-cli-templates/blob/main/hello-world/cargo-generate.toml) for all available options.
 
-> **Prefer Rust?** Use `--define backend_type=rust` instead. You'll need Rust installed with the WASM target: `rustup target add wasm32-unknown-unknown`.
+> **Prefer Rust?** Add `--define backend_type=rust` to the command. You'll also need Rust installed with the WASM target: `rustup target add wasm32-unknown-unknown`.
 
 > **Backend only?** Use a language-specific template instead: `--subfolder rust` or `--subfolder motoko`. These templates have no frontend.