docs: simplify public documentation

[dcajal]

Summary

• reduce the public site to Home, Installation, and a direct API reference
• replace hardcoded package versions with links to the latest GitHub release
• move contributor guidance to a concise root CONTRIBUTING.md
• make root CHANGELOG.md the single release-history source and derive GitHub release notes from it
• remove duplicated module indexes, maintenance pages, and unused documentation plugins

Impact

Users get a shorter path from installation to function documentation and executable examples. Contributors retain the development and release information they need without exposing project-maintenance detail as public-site navigation.

Validation

• pytest: 220 passed against the pinned Biosiglib commit
• mkdocs build --strict
• wheel and source-distribution build; CHANGELOG.md verified in the source distribution
• release-note extraction and workflow YAML validation
• git diff --check