Adding new :deprecation: and :removal: categories

[eric-wieser]

Over at pygae/galgebra, a lot of my release notes are of the form The function :func:useless_function was deprecated. Since these are neither features nor bugfixes, we're currently categorizing them as :support:.

Would it be possible to introduce some more categories? Specifically, I'm finding myself wanting:

• :deprecation:, for functions which have been changed to emit a warning upon use.
• :removal:, for features which have been removed (typically those marked with :deprecation: in a previous release).