Add sphinx fresh environment flag

[robertapplin]

Description of work

This PR adds the sphinx fresh environment flag -E to our document builds. https://www.sphinx-doc.org/en/master/man/sphinx-build.html

This is necessary because when you do not use a fresh environment, if you push two commits in a row, containing the same doctest failure, the second push will not run the failing doctest because the document hasn't changed in the cached environment. Therefore the tests will pass, even when there is a doctest that fails. This assumes both commits run on the same node. This PR will hopefully stop doctest warnings creeping into main!

The only problem with using -E, is it does a rebuild each time, so the doctests will take longer to run. This shouldn't be a problem when we separate the doctests into its own pipeline, and run it on the linux nodes.

There is no associated issue.

To test:

Check that the doctests job passes

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