docs: update Linux install instructions for download.docker.com

[joe0BAB]

Point install docs at Docker's official package repository (download.docker.com) with apt and dnf instructions, note packages can also be downloaded directly from the releases page, and drop the early-access framing.

[docker-agent]

Assessment: 🟢 APPROVE

This is a documentation-only PR updating the Linux install instructions for README.md. The changes are clear, well-structured, and technically sound:

• The curl -fsSL https://get.docker.com | sh -s -- --setup-repo convenience script is the correct Docker-documented approach for repository setup.
• apt-get update is correctly included before the apt install step; dnf correctly omits an explicit update (it refreshes metadata automatically).
• Package names are consistent between the install and uninstall sections.
• The systemctl --user daemon-reload → enable --now ordering is correct.
• The "Set up Docker's package repository" section is logically placed before "Install".
• The uninstall section correctly covers both apt and dnf with matching package names.

No bugs or issues introduced by this PR.

ℹ️ Note: The automated drafter agent did not complete its review (returned review_complete: false with no findings or summary). The assessment above reflects a manual analysis of the diff.