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This is still leftover from the days when the libc++ and libstdc++ formatters were both written in python and in separate categories. Since then we group libstdc++ and libc++ formatters into the same cateogry. This patch removes references to the obsolete `gnu-libstdc++` category from the docs (and a test). See [this thread](https://github.com/llvm/llvm-project/pull/140761#discussion_r2102386080) for more context