Doc: APM setup in Kibana

[juliamrch]

Checking for existing issue

• I have checked for existing issues

Describe the content you'd like to see

The issue

On new Elastic add-on creation, with Kibana and APM activated, Kibana UI won't load the connected services in the APM menu. It will rather ask the user to "Install APM" on Kibana. The configuration wizard will then launch and offer a set of options, that don't actually matter at all: clicking on next and finishing the "installation" is enough to finally launch the list of connected services monitored by the APM server.

The solution

Until this behavior changes, this needs to be documented in the Elastic pages, to avoid getting users lost when they try Elastic x APM for the first time.

[cnivolle]

Related the #333