Make unstable commit versions visible somewhere

[lihaoyi]

Would be nice to not have to dig through the Publish Artifact job logs to figure out what unstable commit versions are available. Even just displaying the published version in the job markdown report may be enough

[lefou]

What I'd like would be to append some notes to the Github Release page of the previous stable release. Or we could maintain a Wiki page, if that is easier to automate. If I'm not mistaken, the wiki is an regular git repo. (Or was that only the case for gitbucket?)

[lihaoyi]

Updated the docs to link to https://repo1.maven.org/maven2/com/lihaoyi/mill-dist, which I guess is the authoritative list of what versions are out there unstable or otherwise, and it gets updated automatically. It's not super pretty, but I guess for this kind of superuser feature it doesn't need to be.

[lihaoyi]

Also added a badge to the front page that shows the unstable version and links to maven central