Add initial support to represent diagrams with Mermaid

[dplanella]

Adds support for generating diagrams with Mermaid for easier maintenance and for lowering the barrier to documentation contributions.

Notes:

• I only replaced one diagram for now to gather feedback. If this change is accepted, happy to convert the rest to Mermaid as well.
• The diagram can be displayed top-down as well (less compact, better for mobile view) by replacing the graph LR line by graph TD

[acs]

Pretty cool. I need to play with mermaid to understand its full capabilities. Thanks @dplanella

[valeriocos]

-1 , I would prefer to replace the diagrams with artworks (pngs files derived from svg ones) as proposed at: #6

[dplanella]

Thanks for the reviews. The ideas behind it were to ultimately lower the barrier for contribution, as:

• Bitmaps are not accessible
• Not everyone feels comfortable with using an SVG editor, but everyone can use a text editor to generate or update the diagrams
• Enables contributors to edit the diagrams online with the GitHub web UI.
• The diagram at [grimoirelab] Restyle grimoirelab-all-complete image #6 could still be used as a nicer representation of how all the tools/backends work together, as an overview on the tutorial landing page.

[dplanella]

@acs @valeriocos @jgbarah any further thoughts on this PR? Thanks.

[valeriocos]

I'm fine with any decision @dplanella :)