docs: separate sandbox installation from first run

[dvdksn]

Summary

Split Docker Sandboxes installation from the first-session walkthrough. The installation guide documents both Linux paths: installing SBX with Docker Engine or installing SBX alone.

@netlify /ai/sandboxes/install/

Preview

• Installation guide
• Get started guide

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[docker-agent]

Assessment: 🟢 APPROVE

The PR cleanly separates installation from the first-session walkthrough. The new install.md page is well-structured, covers all three platforms, and the two Linux paths (combined Docker Engine + SBX install via SBX=1, and SBX-only via REPO_ONLY=1) are clearly distinguished. Cross-reference links are updated correctly, front matter is complete, and no redirects are needed (no pages were removed).

No issues found.

[docker-agent]

Assessment: 🟡 NEEDS ATTENTION

This PR cleanly separates Docker Sandboxes installation content from the first-session walkthrough, creating a new install.md page. The structure is clear, style violations are minimal, and the redirect concern (missing aliases:) is not applicable since the old page was not deleted—only content within it moved.

One item needs confirmation before shipping: the new SBX=1 convenience script flag (see inline comment).