Add Kubernetes Updates docs

[eliaoggian]

I added two pages to document the Update/Upgrade Policy of the Kubernetes clusters.

[github-actions]

preview available: https://docs.tds.cscs.ch/187

[msimberg]

@eliaoggian I made comments on both the policy documents and the main kubernetes page in #188. Perhaps it's easier for reviews if we only keep #188 and merge all in one go?

[bcumming]

Thanks for the contribution @eliaoggian!

Before reviewing the documentation, I first want to understand who the target audience is. In their current form users might expect that this service is available to all users on Alps (which I don't think it is?)

I see the following steps:

1. Decide who is the target audience, and how they fit in with the docs.cscs.ch layout
2. Move the docs to services (or maybe somewhere else, depending)
3. Create a top level "Kubernetes" page that gives an overview of what the k8s service is, and who it is for.
4. Add the sub-pages in the new location.

[eliaoggian]

Moved changes to the kubernetes-docs branch. Closing this PR.