News item: group by categories? or remove non-Oscar.jl news?

[fingolfin]

We used to have too few news items and hence added as much as possible. Now perhaps we have too many: tons of AA, Nemo, etc. releases crowd out the Oscar.jl releases.

Perhaps we should add another field to the metadata of each news item that indicates which category (so: the package name, or "other" ?), and then use that to group the news items; or perhaps to allow only showing those relevant to e.g. Oscar.jl (or any other category).