Add architecture section

[valeriocos]

This PR proposes a new section in the tutorial, called Architecture, which describes the GrimoireLab components. Some parts such as examples and some component descriptions (cereslib, sigils, kibiter) are empty, but they could be addressed by GCI students (probably the examples).

The idea of this PR is also to help refocusing some GCI tasks targeting the tutorial (cc @jsmanrique).

Please @jgbarah have a look, thanks