Move Documentation/etcd-mixin files

[nate-double-u]

Issue

Migrating the Documentation to the website caused the kube-prometheus build to break.

At issue in the kube-prometheus case was the removal of the Documentation/etcd-mixin files. Restoring them solved the kube-prometheus break, but as these files are in the Documentation folder they shouldn't be relied upon for builds.

Solution

Organize with the kube-prometheus and remove the Documentation/etcd-mixin from the Documentation folder.

[nate-double-u]

/cc @underrun @rohitdurvasula @spzala

[ptabor]

https://github.com/etcd-io/etcd/tree/master/contrib looks like a place for this-type of 'integration' code.

[paulfantom]

Hi, kube-prometheus maintainer here 👋

Most projects store their mixins in jsonnet/mixin/ (for example prometheus-operator) or mixin (for example thanos) directories, but usually contrib is also a good place.