blog post categories are confusing and inconsistent

[SethTisue]

I've never understood what the difference between "Blog", "News", and "Announcement" is supposed to be at https://www.scala-lang.org/blog/

and worse, we have published some posts that aren't in any of the three categories — in the root _posts directory, so that's a fourth non-category category

[SethTisue]

overlaps with #849 (that ticket was opened specifically about our RSS feed, but the general issue is broader than that)