[docs] Update structure of Deployment Server pages

Currently in the Deployment Server (DS) documentation is not clear which pages we have. Not from all pages you can return back to the parent page or to the next page.

The structure of the documentation should be changed:

1. Use DS description + assumptions as "main" page for DS
2. At the end of the main page add a list with links to the other DS docs
   Deployment Server Installation
   API Reference
   HTTP Server Setup
   Package Upload
3. Move "Installation" to a separate page
4. Add a reference to the parent page (DS)
   navigation.landingpage = "Qt Automotive Suite Deployment Server" to qdocconf