Improve getting started documentation

[pleonex]

Is your feature request related to a problem? Please describe.
The current documentation can be hard to understand, especially for beginners. We should improve it and structure better.

Describe the solution you'd like
Divide the documentation in several files / topics and explain the proposed architecture. The goal is that developers understand the architecture Yarhl propose, so later the APIs makes more sense.
It would be good to divide each API and include code snippets by topics (binary, nodes, formats). And how it relates with plugins and other tools like SceneGate.
Also change the template so the web looks better and not that old.

Describe alternatives you've considered
Move to other docuementation generators but they lack features or only run on Windows.