Simplify AddSphinxTarget.cmake by just including `GNUInstallDirs`

See the discussion in D30906 for how it must be robust to downstream
projects that might not use `GNUInstallDirs`.

I think an easier approach is just to include `GNUInstallDirs` so that
we can always rely on it. Looking at its definition in
https://gitlab.kitware.com/cmake/cmake/-/blob/master/Modules/GNUInstallDirs.cmake
it appears safe to include multiple times.

I am trying this out because we will face the same problem many times
over if we use `GNUInstallDirs` more widely, as proposed in D99484 and
its parent revisions, since we do not know what downstream projects do
and rather not break them.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D115601