[DOCS] Infrastructure monitoring section needs consistent and standard instructions for installing Elastic stack

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

This is currently inconsistent with similar content in APM and Uptime, but I think both variants have good and bad points: both seemed good in parts and confusing in others.

It would be good to tie this section into the work that @debadair is doing on providing a clear and consistent set-up experience, and also to ensure that new users are presented with a prominent option to get started easily using a free cloud trial.

I've separated the Elastic and Kibana installation out from the Beats installation (also in this topic), because this part is common to all the Observability products. I think it should be possible to produce a single common set of instructions, which can then eventually be reused by all Observability products.