Local dev environment relative pathing (``/`` instead of ``/en/latest/``)

[jkwmoore]

The live website is hosted under /en/latest/ not / which is what the test environment will host when building / livehtml. which causes test failure with with success in prod or vice versa.

This should probably be corrected in the Makefile, https://github.com/rcgsheffield/sheffield_hpc/blob/master/Makefile

N.b. this will likely need amendments in both the above and the GitHub build action.