document how to use installations for a different CPU microarchitecture / GPU builds

[trz42]

Initialising the EESSI environment is described on https://www.eessi.io/docs/using_eessi/setting_up_environment/

This automatically chooses the installations providing the best performance for the system to be used.

However, it might be interesting to test installations for a different architecture. For example, setting EESSI_SOFTWARE_SUBDIR_OVERRIDE=x86_64/generic before initialising the EESSI environment would give access to generic builds for x86_64 CPUs.

Documentation could be extended to guide users how to achieve this (and also highlight potential issues such as using installations that are not supported due to incompatible CPU family or microarchitecture).