docs: add documentation for keptn.sh/container annotation

[norman-zon]

Summary

If a pod contains multiple containers the user can select which container the version should be extracted from by setting the keptn.sh/container annotation. If the annotation is set, but no container matches its name we assume something is wrong an throw an error.

Fixes #2484

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[norman-zon]

@mowies , I am not too sure about appending the container annotation to the list, because there is no app.kubernetes.io/ equivalent. But it still seems to be the best place to put it for visibility

[StackScribe]

Nicely done! I left a couple small editorial suggestions but otherwise, this is ready to go!

[thisthat]

I left some comments to make the lint checks for MD files passing :)
Otherwise, LGTM content-wise :)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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