CI: Sphinx exec Haskell directive checks output against golden file or cache

The sphinx extension we use to run haskell code should check the output of that code against a golden file to ensure that the book's code snippets build in the way we expect and to ensure output does not change across GHC/Cabal/Stack versions.

The code is in `root/extensions/sphinx_exec_directive.py`. We should have:

- A setting that points to the golden file directory. This directory layout should follow the layout of the book.
- A setting exposed to the user that disables this feature in `conf.py`. We want fast rebuilds while writing and checked builds on CI.