Add first Kubernetes deployment guide

[kirstenschwarzer]

This PR adds a first Kubernetes deployment guide to the documentation site.

[TeganAli]

@kirstenschwarzer I've tried my best to update text to match the UI, based on the screenshots, but not everything is included in the screenshots.
It's not critical, but as per our style guide, it's ideal if we can match the UI when we're documenting what people should see as they follow the process :)

[TeganAli]

This docs page is really well written though, so hasn't needed many changes at all :)
I'll review again on Monday with fresh eyes and let you know when I'm done.