docs: consolidate documentation structure

[drew]

Summary

Consolidates the docs navigation and merges overlapping overview pages into fewer, clearer documentation surfaces.

Related Issue

None.

Changes

• Merge gateway and sandbox overview content into the main Sandboxes and Gateways pages.
• Move Inference Routing under How It Works and collapse the separate inference pages into a single page.
• Move Architecture into a dedicated How OpenShell Works page under About, with a shorter Mermaid-based architecture section.
• Document local machine and remote Kubernetes deployment models on the How OpenShell Works page.
• Document supervisor protection layers for process, filesystem, network, credentials, inference, and observability boundaries.
• Update Gateway Authentication with OIDC support and generic mTLS environment-variable setup.
• Rename Get OpenShell to Installation and update internal links.
• Remove standalone About Gateways and Sandboxes, Community Sandboxes, Architecture, and inference section pages.
• Update navigation, homepage cards, and cross-links to the new page locations.

Testing

• mise run docs:build:strict passes
• mise run markdown:lint passes
• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

[github-actions]

🌿 Preview your docs: https://nvidia-preview-pr-1231.docs.buildwithfern.com/openshell

[TaylorMutch]

Should this link to the helm chart readme?