docs: expand documentation site and add llms.txt generation

[rororowyourboat]

Summary

• Add documentation pages for stockflow, control, and software packages (API refs, guides, getting started)
• Add cross-cutting guides: best practices, DSL comparison, troubleshooting
• Add Evolution of Trust example and update feature matrix
• Add mkdocs-llmstxt plugin to auto-generate /llms.txt (index) and /llms-full.txt (full content) on every build — 162 pages across 11 sections

Test plan

• uv run mkdocs build --strict passes
• uv run ruff check packages/ passes
• uv run ruff format --check packages/ passes
• site/llms.txt and site/llms-full.txt generated with correct content