Update documentation to include non-NCI/Gadi use cases

[marc-white]

Is your feature request related to a problem? Please describe.

#357 has pointed out that there are use cases of access-nri-intake-catalog beyond Gadi - specifically, the ability to tie the custom builders in with intake-esm to produce datastores of experiments outside Australia.

Describe the feature you'd like

A new section should be added to the documentation, providing instructions for this use case.

Describe alternatives you've considered

Nil.

Additional context

Nil.