docs: collapse Docker install section in README

[BrettKinny]

Summary

• Wraps the "Don't have Docker or Podman? One-line install" subsection in a <details>/<summary> block so it renders collapsed by default on GitHub.
• Reduces visual noise in the README for the common case where the reader already has Docker or Podman installed; the one-liner install steps are still one click away.

Notes for reviewer

• The section is no longer a Markdown heading, so it will not appear in GitHub's auto-generated table of contents. If we want it back in the TOC, we can nest a ### heading inside the <summary> element.

Test plan

• Verify the rendered README on GitHub shows the section collapsed by default with a clickable disclosure triangle.
• Expand it and confirm the macOS / Linux / Windows install snippets render correctly.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Updates the README to reduce clutter by collapsing the “Don’t have Docker or Podman? One-line install” instructions behind a GitHub-rendered <details>/<summary> disclosure, while keeping the install snippets easily accessible.

Changes:

• Replaces the Markdown ### heading for the one-line Docker/Podman install subsection with a collapsed <details>/<summary> block.
• Keeps the macOS/Linux/Windows install snippets unchanged, only altering presentation/structure.

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