Write instructions for updating Spack

[penguian]

Right now there are instructions for setting up Spack, but they are part of ACCESS-OM2 build instructions and are therefore hard to find.

In the instances where someone has already set up Spack and wants to update their local spack or spack-config repository there are no instructions on how to do so and keep these repositories in sync.

Add such instructions to https://github.com/ACCESS-NRI/dev-docs/wiki/Spack

[anton-seaice]

I think the first point has been addressed through https://access-hive.org.au/models/run-a-model/build_a_model/