2.11.1: standalone build (fin-primitives from crates.io), README and …

…docs.rs lead with a live tape GIF, install table and 3-step program (#3)

- Depend on fin-primitives = "2.14" instead of ../fin-primitives, so a plain
  clone builds; CI no longer checks out the sibling repo.
- README top: one-sentence summary, a real-speed recording of the tape example,
  install table, a 3-step program (doctested with the rest of the README) and
  its real output, then results. Removed the stale 'crates.io has 2.4.2' note.
- docs.rs front page: summary, GIF, cargo add line, links to the main types.
- Cargo metadata: clearer description, keywords, categories.

ci: replace with minimal cargo check pipeline

[infra] add .gitignore, remove target from tracking