docs: add rust_icu hello world example

[arnavnagzirkar]

Adds a concise Hello World section to the README showing how to use rust_icu_umsg MessageFormat with rust_icu_common, rust_icu_uloc, and rust_icu_ustring dependencies.\n\nThis is a docs-only change.

[filmil]

How about an actual "examples subdir with the configurations you want to demo?

[arnavnagzirkar]

Good call — done. I moved the demo into a standalone, runnable examples/hello_world crate:

• examples/hello_world/Cargo.toml — the exact dependency configuration to demo (rust_icu_common, rust_icu_uloc, rust_icu_umsg, rust_icu_ustring), with a comment showing the crates.io version alternative for downstream users.
• examples/hello_world/src/main.rs — cargo run prints Hello, World!.
• examples/hello_world/README.md — how to run it.

The crate is added to exclude in the root Cargo.toml so the CI feature matrix (cargo test --no-default-features --features ...) is unaffected, and the top-level README now links to the example instead of inlining the code.

[filmil]

Is this something that could be tied into presubmits? It would be much more useful if it's actually checked that it works.

[arnavnagzirkar]

Good idea. I have wired the example into the presubmits so it is actually verified on every push and pull request.

What I changed:

• examples/hello_world/src/main.rs now puts the formatting logic in a greet() function with a #[test] that asserts the output equals Hello, World!, so the check verifies correctness and not just that the code compiles.
• A new docker-test-example target in the Makefile builds and runs the example inside the same dockerized test environment the rest of the suite uses. It runs cargo test (the assertion) and then cargo run.
• A new test-examples job in .github/workflows/test.yml runs that target across ICU 74, 76 and 77, matching the existing default feature matrix.

Because the example depends on the in tree crates with their default features, I kept it on the default feature configuration rather than the --no-default-features --features ... matrix, which targets the library crates. If the example ever stops compiling or stops printing Hello, World!, CI now fails.

You can reproduce the presubmit locally with:

make DOCKER_TEST_ENV=rust_icu_testenv-77 RUST_ICU_MAJOR_VERSION_NUMBER=77 docker-test-example