Docs: Add templating documentation to mkdocs navigation

[tomdurrant]

Summary

Complete documentation restructuring and enhancement for rompy. This PR introduces a new docs architecture with improved navigation, comprehensive guides, and new content.

Major Changes

New Documentation Structure:

• Reorganized navigation in mkdocs.yml with clearer sections
• Added progressive tutorials for learning rompy step-by-step
• Created comprehensive model guides (SWAN, SCHISM, XBeach, WW3)
• Added architecture overview and schema advantages documentation
• New testing guide and contribution guidelines

New Content:

• getting_started.md - Quick onboarding guide
• progressive_tutorials.md - Step-by-step learning path
• examples.md - Practical usage examples
• common_workflows.md - Frequently used patterns
• extending_models.md - Guide for adding new models
• faq.md - Frequently asked questions
• configuration_deep_dive.md - Advanced configuration patterns
• templating.md - Template variable substitution documentation
• testing_guide.md - Comprehensive testing documentation
• Model-specific guides: swan_guide.md, schism_guide.md, xbeach_guide.md, ww3_guide.md

Updated Content:

• index.md - Refreshed introduction and overview
• core_concepts.md - Enhanced explanations
• contributing.md - Expanded contribution guidelines
• api.md - Updated API reference
• plugin_architecture.md - Improved plugin documentation
• backends.md - Enhanced backend documentation

Removed:

• quickstart.md - Replaced by getting_started.md
• installation.md - Merged into getting_started.md
• Redundant/duplicate pages

Infrastructure:

• Updated mkdocs.yml navigation structure
• Added mkdocs-jupyter for notebook integration
• Updated GitHub Actions workflow for docs building
• Added notebook gallery structure

Files Changed

• 40 files changed, +3,931 insertions, -292 deletions

Testing

• All configuration examples validated
• Docs build successfully with mkdocs
• Links verified

Ready for review!