Update Logs + Infrastructure docs to specify shipper compatibility and index customization

If you look at the forums, as well as support cases, shipper compatibility with the new Infrastructure and Logs UI is one of the most frequent questions around getting started.
https://discuss.elastic.co/c/infrastructure
https://discuss.elastic.co/c/logs

Looking at our documentation, we are missing sufficient detail:
https://www.elastic.co/guide/en/infrastructure/guide/current/install-infrastructure-monitoring.html

At a minimum, I think we should add information around:
1. The fact that shippers of versions 6.5 and above are require
2. That users can customize in kibana.yml which index(es) this Logs and Infrastructure UIs looks at.

cc @alvarolobato @makwarth

Start with the install-infrastructure-monitoring.html page linked in the issue and review its existing shipper setup guidance. Verify the stated 6.5+ shipper compatibility and the kibana.yml index customization details. Done means the Logs and Infrastructure documentation clearly explains both requirements.

Written by the indexing model from the issue text.