Reorganise page and menu layout to be more useful to users

[aturner-epcc]

Some of the documentation sections are too long to be easily digestable and useable by users. We could reorganise into a structure such as that used by NERSC:

https://docs.nersc.gov/

This would also be a good first step to moving towards "docs.epcc.ed.ac.uk" to cover all HPC systems - maybe even EIDF too