Split glasgow_exts into several files (#17316)

This patch breaks the huge glasgow_exts.rst file into several files.

The split isn't exactly as discussed in #17316 (closed) for now. However it's a step in the right direction. It will be much easier to rearrange sections after this patch as we will just have to modify the TOCs.

The rendered docs can be previewed here: https://vrac.hsyl20.fr/ghc-users-guide/exts.html