Rename `packages.md` to `README.md` so Github shows it automatically

[evansd]

If we renamed the packages.md files to README.md then Github would display them automatically when viewing the v1, v2` etc directory which might give a slightly nicer UX.

I know we link to them directly from the root README, but I've just seem one user tell another to go the docker image repo and look at the v2 directory and they'll have to click around a bit before they discover packages.md.

Slack thread:
https://bennettoxford.slack.com/archives/C01D7H9LYKB/p1777284322027509?thread_ts=1777283995.725839&cid=C01D7H9LYKB

[suzannehamilton]

Closing because we think renaming would break the existing links, and there's no way to add a redirect.

[bloodearnest]

Speculative symlink PR seems to work: #127