Internal RTD /en/latest structure redirect not highlighted in the README

[jkwmoore]

See: https://github.com/rcgsheffield/sheffield_hpc/blob/master/rtd-root-redirect.html

This performs the redirect: https://docs.hpc.shef.ac.uk/ --> https://docs.hpc.shef.ac.uk/en/latest

So we retain the legacy structure we inherited from the ReadTheDocs version of the website (otherwise Google search indexing etc.. would have broken.)