Create reference documentation for upgrades

[jgehrcke]

Currently, https://github.com/NVIDIA/k8s-dra-driver-gpu/wiki/Installation#upgrading is still empty:

Fill this section with the bare minimum.

Ref: https://github.com/NVIDIA/k8s-dra-driver-gpu/blob/08f3d9a29853ed26715fe5a21bc528d428d65920/tests/bats/test_cd_updowngrade.bats#L35

[jgehrcke]

I filled in a first version for this at https://github.com/NVIDIA/k8s-dra-driver-gpu/wiki/Installation#upgrading -- @klueska can you please take a minute to quickly review and edit?