docs: separate mdBook sources and output

[shayyz-code]

Summary

• stop tracking 42 generated mdBook output files
• ignore regenerated docs/book/ content
• document pinned mdBook installation plus canonical build and test commands
• update contributor, agent, and roadmap guidance

Issue

Closes #13

Verification

• mdbook build docs
• mdbook test docs
• oranda build
• cargo fmt --all -- --check
• cargo check --all-targets
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• actionlint

Documentation and visual impact

Documentation output is regenerated rather than tracked; published content and visual presentation are unchanged.

[cursor]

Bugbot is not enabled for your account, so this pull request was not reviewed.

Enable Bugbot in the Cursor dashboard to get automatic reviews on future PRs.