docs: document experimental Docker CE support and known limitations

[joe0BAB]

Replace the Docker CE roadmap note with install/uninstall instructions for the experimental, early-access integration (Docker Engine 29.2.0+), and add a Known limitations section covering multi-user support, the keyring backend requirement, and manual restart after a dockerd restart.

[docker-agent]

Assessment: 🟡 NEEDS ATTENTION

This PR adds clear and helpful documentation for the experimental Docker CE integration. One broken anchor link will silently fail in the rendered README and should be fixed before merge.

[docker-agent]

[MEDIUM] Broken anchor: #known-limitations won't scroll to the section

The WARNING callout links to #known-limitations, but GitHub generates the anchor from the actual heading ## Known limitations and issues as #known-limitations-and-issues. The link will be silently dead in the rendered README — clicking it does nothing.

Fix: Change line 42 from:

[known limitations](#known-limitations)

to:

[known limitations](#known-limitations-and-issues)

[docker-agent]

[LOW] Uninstall section omits systemctl --user daemon-reload after apt remove

The install section explicitly runs systemctl --user daemon-reload to register the new unit file with systemd. The uninstall section skips the equivalent step after package removal, which can leave stale unit metadata in systemd (e.g., the unit may still show as "loaded" in systemctl --user status docker-secrets-engine even after the package is gone).

Suggested fix: Add a daemon-reload at the end of the uninstall block:

systemctl --user disable --now docker-secrets-engine.service
sudo apt remove docker-secrets-engine-plugins docker-secrets-engine
systemctl --user daemon-reload