docs: user guide about graceful pod termination

[viktor-f]

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Fixes #906

This writes about graceful termination of pods and how users can manage that.

[simonklb]

Just some minor things + fix one sentence per line. Other than that, LGTM!

[viktor-f]

Updated now. But I'm not sure if we are trying to follow "one sentence per line" in this repo, since there are a lot of pages that does not do that. I changed it regardless since I agree that it is nice to have.

[cristiklein]

Nice addition, thanks! Indeed, getting Pod termination right is something new when coming to the Kubernetes world.

I'd also like us to go for one-sentence-per-line. I know this is not consistent in this repo, but that's where we will slowly head to.