Fix instructions for running payu at NCI

[aidanheerdegen]

Current docs tell users to

module load payu

This is out of date, and payu should be accessed through the CMS conda environment.

[aidanheerdegen]

Also there are references to raijin that need to be updated:

$ grep -ir raijin *
config.rst:   values may not be correctly communicated across raijin nodes.*
install.rst:as Raijin. If you wish to use payu on other systems, see the notes at the end
install.rst:Payu is available for all users on Raijin.
install.rst:     are based on the OpenMPI implementation. We also rely on Raijin's internal
install.rst:There are also some additional assumptions based on the architecture of Raijin.