Quarto examples and docs setup

[MitchellAcoustics]

This pull request focuses on restructuring the documentation system for Soundscapy, clarifying the build process, and updating links to reflect the new documentation architecture. The main changes include implementing a two-site documentation split (reference/docs and tutorials/user guide), updating references and navigation to point to the new URLs, and improving configuration for documentation builds and formatting.

Documentation architecture and navigation:

• Added a comprehensive restructure plan in docs/_plan.md, outlining a two-site split: Zensical (MkDocs Material) for main docs/reference and Quarto for tutorials/user guides, with detailed content mapping and future actions.
• Updated all internal documentation links in docs/background.md and docs/index.md to point to the new published Quarto tutorials URLs instead of local rendered files, ensuring users are directed to the correct live documentation. [1] [2] [3]
• Replaced the contents of docs/tutorials/index.md with a redirect notice to the new Soundscapy User Guide, reflecting the migration of tutorials to the Quarto site.

Build and workflow improvements:

• Updated the GitHub Actions workflow (.github/workflows/docs.yml) to clarify that the documentation build step now covers both Zensical and Quarto tutorials, matching the new two-site architecture.
• Improved .pre-commit-config.yaml to exclude Quarto cache and requirements directories from formatting hooks, preventing accidental formatting of generated or dependency files.

Project structure and housekeeping:

• Updated examples/.gitignore to properly ignore Quarto project cache directories while ensuring that freeze outputs remain tracked for CI builds.