docs: add verified usage and development guides

[egorsmkv]

The README lacked a complete extraction example and the parse rustdoc still recommended the legacy unsafe owned constructor. Add a current-source quickstart, feature table, usage guide and development guide covering ownership, attributes, selector rejection, decoded text, bounded capacities, migration and HTML5 limitations.

Include the usage guide in crate rustdoc so its Rust examples run as doctests. Update the owned-input recommendation to VDomGuard::parse. Extend CI path filters to docs/** and add strict default/entities rustdoc builds. Parser behavior and dependencies are unchanged.

Validation on Rust 1.99.0:

• cargo test --doc: default (12), std (16), entities (16), all passed, including compile-fail cases.
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps: default and entities passed.
• cargo fmt --check, git diff --check and repository documentation link checks passed.

The guide distinguishes current Git APIs from published 0.2.0; this PR does not bump or publish the crate version.