Refactor ShARC GPU page intro heading and paragraph.

[jkwmoore]

At the moment the initial paragraph makes it sound like you get access via purchasing a node or by being part of a research group that has done.

We should probably re-work this to be more clear about public nodes being available but that some users will have access to restricted resources:

https://github.com/rcgsheffield/sheffield_hpc/blob/master/sharc/GPUComputingShARC.rst#requesting-access-to-gpu-facilities