Edit and restructure narrative docs on Integrations

[alaudazzi]

This is part of part of #3678

Problem

As reported by PMs:
"Today our documentation does not put a lot of emphasis on the OOTB experience and the information is sparse and sprinkled in various places. (...) We should lead with OOTB and this should be front and center of the capabilities that Elastic offers, highlighting how easy it is to get started."

Holistic strategic solution - Suggested doc structure

As suggested by PMs:
"High level intro to everything that comes out of the box today. It doesn’t require YOU to do the work. We’ve done the work for YOU, so you can start using all the OOTB (pre-built stuff) right away! We need to sell this. Suggest we take it out of the Manage Elastic Agent integrations section (which sounds more like work that the customer has to do, invest money and time in)
Can we have an OOTB section that talks about everything that the customer is getting pre-made (ready to use). What you need to do to get the asset (eg you can’t get the asset unless you install the integration)
Introduction to OOTB alert templates and how to use them (can elastic/kibana#221633 be improved?)
FAQ -
how do we help customers understand the difference between OOTB OOTB alert rules and alert templates for integrations (which are essentially OOTB alert templates)
Be clear and concise about typical customer questions. An FAQ format is better than long text"

Example of restructured docs: #3461