Expand examples beyond hello world

[darthfork]

Examples are part of the product for Bazel rulesets.

Tasks:

• Add a multi-package example with zig_library and zig_binary.
• Add a test example.
• Add a cross-compilation example if supported.
• Add a Bzlmod consumer example.

Acceptance criteria:

• examples/ covers common usage patterns.
• README points users to the examples.

Parent issue: #4