[build2] Changes to package review/transition policies on cppget.org

[Boris Kolpackov]

We are inviting the build2 community to help with reviewing package
submissions. We are also adjusting the package transition policies
between queue.cppget.org and cppget.org as well as between sections
within cppget.org. Specifically, migrating a package from
queue.cppget.org to cppget.org no longer requires a review, only
that the package builds in at least one configuration. As a result,
the alpha, beta, and testing sections of cppget.org may now contain
unreviewed packages. However, a package can only be migrated from
testing to stable if it passes a review.

See the full announcement for details, including the review procedure:

https://build2.org/blog/package-review-transition.xhtml

Let me know if you have any question or concerns.