docs: separate mdBook sources and output

[shayyz-code]

Goal

Keep mdBook sources authoritative, remove generated output from Git, and define one reproducible local documentation build.

Scope

• Stop tracking and ignore docs/book/.
• Document mdBook 0.5.2 installation plus canonical build and test commands.
• Update contributor, agent, and roadmap guidance.
• Preserve the existing Oranda/GitHub Pages workflow.

Acceptance criteria

• mdbook build docs regenerates ignored output from docs/src/.
• mdbook test docs and oranda build pass.
• Repository formatting, compilation, strict Clippy, tests, and workflow lint pass.
• No documentation prose or language syntax changes.