Fix download links in docs (#3205)

Not sure when it ended up broken, but the github URL format doesn't
match the URLs generated (missing a `/download/` segment), and
`millVersion` is incorrectly providing the unstable path even for
commits for which no unstable release was published

build.sc

@@ -1230,7 +1230,7 @@ object example extends MillScalaModule {
  val title =
  if (seenCode) ""
  else {
- val label = millVersion()
+ val label = millLastTag()
  val exampleDashed = examplePath.segments.mkString("-")
  val download = s"{mill-download-url}/$label-$exampleDashed.zip[download]"
  val browse = s"{mill-example-url}/$examplePath[browse]"
@@ -1715,7 +1715,7 @@ object docs extends Module {
  | mill-github-url: ${Settings.projectUrl}
  | mill-doc-url: ${if (authorMode) s"file://${T.dest}/site" else Settings.docUrl}
  | mill-download-url: ${if (authorMode) s"file://${exampleZips().head.path / os.up}"
- else s"${Settings.projectUrl}/releases/latest"}
+ else s"${Settings.projectUrl}/releases/download/latest"}
  | mill-example-url: ${if (authorMode) s"file://${T.workspace}"
  else s"${Settings.projectUrl}/blob/main/"}
  | utest-github-url: https://github.com/com-lihaoyi/utest