docs: link out to container engine install from health

[awphi]

Changes

• As title says
• Renames 'Install Docker' to 'Install a Container Engine' in README.md since it needs to be a bit more future-proof given the CLI links to it now. Considered using perma-links generated in the CLI but felt kinda nasty.

Host
----
Topo: ✅ (topo)
OpenSSH: ✅ (ssh)
Container Engine: ❌ ("docker" not found in $PATH)
  Fix:
    Install a supported container engine. See https://github.com/arm/topo#install-a-container-engine

Host
----
Topo: ✅ (topo)
OpenSSH: ✅ (ssh)
Container Engine: ✅ (docker)
Docker Compose: ❌ (local command failed: exit status 1 | stderr: docker: unknown command: docker compose

Run 'docker --help' for more information
)
  Fix:
    Ensure Docker Compose is installed as a plugin for Docker. See https://github.com/arm/topo#install-a-container-engine

Target
------
ℹ️ provide --target or set TOPO_TARGET to check target health

Checklist

• 🤖 This change is covered by tests as required.
• 🤹 All required manual testing has been performed.
• 📖 All documentation updates are complete.