Provide a guide how to use the HTTP Tracking API

[tsteur]

When going to the Integrate page we see a couple of Tracking guides. The first two links "JS Tracking Client" and "Tracking API Clients" list clients that access the HTTP Tracking API. The third link is a guide how to use Content-Tracking via the HTTP Tracking API but there is actually no guide how to use the HTTP Tracking API which would be helpful if people want to write a client or if they want to track data in their application without using any of those clients.

I don't think we need a huge guide here but we need to explain some basics and the link to the Tracking API Reference.

At the end of the API Reference there is for example one part "Debugging the Tracker" which could be part of the guide. How to construct an example request could be moved there as well and explained in steps. We also need to explain when authentication is needed and how they can test it etc.