[DOCS] infrastructure monitoring section needs clearer instructions for installing and configuring required Beats

[Titch990]

Topic https://www.elastic.co/guide/en/infrastructure/guide/current/install-infrastructure-monitoring.html

The instructions for installing and configuring Beats need clarifying, testing and checking. In particular, I think there is some overlap between the in-app instructions in Kibana (which is one of the options mentioned here), and the direct instructions here, which could cause confusion for some readers.

Any work carried out in this area should probably bear in mind the imminent separation of Infrastructure and Logs content, but also the changes likely to occur as a result of the integrations work, which may bring parts of this together again?

As part of working on this issue, I'd like to better understand the current installation and configuration, and the changes likely as a result of the modules work. Then it will be possible to decide what is sensible to fix now, to improve the current user experience, and what should wait until later.

@Titch990