Review/remove HPC-related content in docs

[alexdewar]

I haven't reviewed this content, but I suspect it's probably out of date. For one thing, it tells users to use EasyBuild to get HealthGPS and the only version of HealthGPS on EasyBuild is really out of date. (IMO it's probably not worth trying to fix the EasyBuild recipe either -- the PR to merge it has been open for years.) Besides that it also has a bunch of instructions on how to submit an HPC job etc. which, aside from likely being specific to Imperial's HPC, is really not the right place for that sort of documentation, as it's nothing to do with HealthGPS. If HealthGPS users want help getting started with the Imperial HPC, they should refer to the RCS docs instead.

I suggest we ditch all of it or at least most of it. The only HPC-specific thing I think we need to flag to HealthGPS users is that they may want to set the maximum number of threads, but nothing else comes to mind.

What do you think @jamesturner246?