[docs] Add example tracing infrastructure (#1866)

This adds an example on how to get Grafana Tempo up to receive spans as
well as Grafana itself to view them.

I've added this as a separate Tracing doc in the installation guide as
the Advanced one was starting to get rather full.

Fixes: #1791

example/tracing/grafana-datasources.yaml

@@ -0,0 +1,16 @@
+apiVersion: 1
+
+datasources:
+- name: Tempo
+  type: tempo
+  access: proxy
+  orgId: 1
+  url: http://tempo:3200
+  basicAuth: false
+  isDefault: true
+  version: 1
+  editable: false
+  apiVersion: 1
+  uid: tempo
+  jsonData:
+    httpMethod: GET