Consider using `graphviz` instead of `mermaid`

[alnoki]

Presently, Econia's Move code documentation comments use fenced code blocks with mermaid.js syntax for custom dependency charts.

Move DocGen dependency charts use graphviz, however, a similar tool. For consistency it is suggested that future documentation comment fenced code blocks use graphviz syntax.

[alnoki]

Closing per ease of maintaining dependency charts solely via Move DocGen