Please add information about how to log into the playground dashboard and granana instances

[bkane-msft]

File: /release-8.5/tiup/tiup-playground.md

This would also fit will into https://docs.pingcap.com/tidb/stable/tiup-overview/

From https://www.pingcap.com/blog/distributed-sql-tutorial-first-steps-setting-up-tidb-locally/ I was able to use admin/admin to log into grafana but the root/<empty password> didn't work for the dashboard

[ti-chi-bot]

Welcome @bkane-msft! It looks like this is your first issue to pingcap/docs 🎉

[bkane-msft]

It would also be super nice if the playground startup printing simply printed the usernames and passwords for the dashboard and grafana instance in addition to the other information.