docs: add provider connection guides

[killswitch-GUI]

Summary

• add a provider chooser and focused setup guides for Codex/ChatGPT, the OpenAI API, OpenRouter, local models, and other OpenAI-compatible endpoints
• add validated schemaVersion: 2 provider overlays with credential, routing, sandbox, verification, and failure guidance
• group the guides in collapsible Zensical navigation and cross-link the canonical onboarding, operator, and configuration pages
• add parser-backed documentation contracts and CLI route validation

Impact

Users can choose their model access method directly from /use/, follow a copy/paste-ready task path, and collapse the provider subgroup to keep the sidebar compact.

Validation

• cargo test -p colossus-cli --test documentation_contract
• ./scripts/docs-site build
• Playwright verification of expanded and collapsed desktop navigation
• cargo xtask check rust

Closes #93

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