Make the compiler README easier to adopt

[konojunya]

Background

The README explained the compiler pipeline and maintainer checks, but did not clearly identify the intended library audience or show the smallest API integration.

Implementation Approach

• Clarify when to use the compiler crate versus the CLI or Playground.
• Add a copyable compile example that produces normalized IR.
• Link directly to the published docs.rs API.
• Keep release instructions with the maintainer commands.

Testing Approach

• cargo test --workspace
• cargo doc --workspace --no-deps
• Compiled and ran the README example in an isolated Cargo project.
• Checked local Markdown links and whitespace.

Review Perspectives / Points of Concern

Please verify that the example demonstrates the renderer-independent boundary without implying that this crate renders diagrams.