Add Sphinx spelling files to spellfile

[Julian]

I haven't fully thought through how this would work, but it would be nice if spelling files (from sphinxcontrib.spelling) were automatically discovered and added to vim.opt_local.spellfile.

There's a default location for that file, but it's configurable.

(The end goal is for that to then propagate to ltex-ls but that's easy once it's in spellfile)