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.