Write a tutorial for building a dashboard from scratch

[alpgarcia]

The goal of this task is creating a tutorial to guide newcomers through the whole process of creating a dashboard.

Some steps that would be great to cover:

• How to install GrimoireLab to start working. Provide links to the corresponding documentation, not a full explanation.
• Creation of searches. This helps to produce final widgets as tables, or to be later used by other widgets to visualize subsets of the original index pattern.
• Creation of widgets. Connected to the previous point because the visualizations might be created on top of an index, on top of a Search, or by filtering the information directly in the widget with specific filters.
• Creation of dashboards. This is an aggregation of visualizations.

Some guidelines we could provide:

• Use donuts instead of full pie charts for a matter of style.
• Do we really need a pie or a bar chart would be better for comparing purposes?
• Try to create a dashboard that does not need any scroll. Free space is important too to avoid overwhelming the user.
• Try to create widgets without scroll bar inside (this may happen in tables).

PR related: #188