Unify instructions for installing sample data in Kibana

Currently this is documented in 2 places: 

- https://www.elastic.co/docs/explore-analyze#gs-get-data-into-kibana
- https://www.elastic.co/docs/manage-data/ingest/sample-data

We should only keep the 2nd and update all references to the 1st one to the 2nd as well as add a redirect