Use CAPS for string replacement except where bash expansion appropriate

[Carldkennedy]

Suggest we replace $USER with YOUR_USERNAME throughout documentation except where $USER would be an appropriate bash expansion, where possible using $HOME.

$CLUSTER_NAME with CLUSTER_NAME etc

Originally posted by @Carldkennedy in https://github.com/rcgsheffield/sheffield_hpc/issues/2052#issuecomment-2025662407