Move node-mixin into docs directory

Signed-off-by: beorn7 <beorn@grafana.com>

docs/node-mixin/config.libsonnet

@@ -0,0 +1,11 @@
+{
+  _config+:: {
+    // Selectors are inserted between {} in Prometheus queries.
+    nodeExporterSelector: 'job="node-exporter"',
+
+    // Mainly extracted because they are repetitive, but also useful to customize.
+    fsSelectors: 'fstype=~"ext.|xfs",mountpoint!="/var/lib/docker/aufs"',
+
+    grafana_prefix: '',
+  },
+}