docs: refresh 0.3.0 API guidance and parser compatibility

[egorsmkv]

Documentation still described the manifest as unbumped and linked to compatibility and benchmark reports absent from the repository. Update the 0.3.0 guidance, replace broken references and add a compatibility reference covering feature-dependent signatures, CSS support and bounded parse errors.

Add a tested DOM traversal and markup-writing example, with serialization, sink-error and allocation semantics traced to the implementation. No runtime code or dependency changes.

Validation on Rust 1.99.0 against base c282e38:

• cargo test --doc --no-default-features: 13 passed
• cargo test --doc --features std: 17 passed
• cargo test --doc --features entities: 17 passed
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps for default and entities: passed
• cargo fmt --check, git diff --check and local Markdown file-link checks: passed

Nightly SIMD and Miri were not rerun for this documentation-only change.