Various documentation fixes

• Remove UnliftedFFITypes from conf.py. Some time ago, this extension was undocumented and we had to silence a warning. This is no longer needed.
• Use r'' in conf.py. This fixes a Sphinx warning: WARNING: Support for evaluating Python 2 syntax is deprecated and will be removed in Sphinx 4.0. Convert docs/users_guide/conf.py to Python 3 syntax.
• Mark GHCForeignImportPrim as documented
• Fix formatting in template_haskell.rst
• Recursive do is now supported in Template Haskell

cc @Kleidukos

Tracking: #18641