[APM] POC for Fleet integration (new getting started guide)

[sorenlouv]

Related: #89989
The existing Getting Started guide is being replaced by a new one located within Fleet (they will both co-exist until 8.0).

The new Getting Started guide can be created by hooking into the Fleet framework. Look at this PR for details.

What the new guide should contain
WIP

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[graphaelli]

@simitt do you understand this to be for the overview page on integrations installation (app/fleet#/integrations/detail/apm-0.1.0-dev.5/overview) ?

[simitt]

I did not imagine to completely move the Getting Started Guide inside Fleet, at least not for 7.x. Fleet currently requires superuser privileges, but the APM Getting Started Guide does not. For 7.13 we are discussing to only link to the Fleet Integration from the Getting Started Guide and to ensure the APM Server URL + secret token can still be injected into the Getting Started Guide.

Moving forward it might make sense to move more configuration towards Fleet, but I don't see this a requirement for going GA.