[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.

从链接的 Infrastructure Monitoring 安装主题开始，将其中关于 Elastic/Kibana 和 Beats 的说明与相应的 APM 和 Uptime 内容进行比较。审查 issue 中提到的设置体验工作，包括免费云试用选项；完成的标准是形成一套清晰、一致的说明，适合在各个 Observability 产品中复用。

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