docs: integrate guides into mkdocs site as documentation pages

[rororowyourboat]

Summary

The 4 new guides in packages/gds-examples/guides/ are currently standalone Python modules and marimo notebooks. They should be incorporated into the mkdocs documentation site as proper pages so users can discover them through the docs.

Guides to Integrate

Guide	Location	Description
Getting Started	guides/getting_started/	Progressive 5-stage "Build Your First Model" tutorial
Rosetta Stone	guides/rosetta/	Same model across raw GDS, stockflow DSL, and control DSL
Verification	guides/verification/	Deliberately broken models demonstrating all 3 verification layers
Visualization	guides/visualization/	gds-viz feature showcase with themes and cross-DSL views

Tasks

• Add guide pages to mkdocs.yml nav structure
• Create markdown wrappers or adapt existing content for mkdocs rendering
• Cross-link from API reference pages to relevant guides
• Add a "Guides" section to the docs landing page
• Verify all Mermaid diagrams render in the mkdocs theme