add redirect for GPU docs

[branfosj]

#188 moved docs/gpu.md to docs/site_specific_config/gpu.md.

But loading CUDA gives the message

$ module load CUDA/12.1.1
Lmod has detected the following error: 
You requested to load CUDA  but while the module file exists, the actual software is not entirely shipped with EESSI due to licencing. You will need to install a full copy of the CUDA SDK
where EESSI can find it.
For more information on how to do this, see https://www.eessi.io/docs/gpu/.