Add graphical visualization of `Diagram`

[maloleroy]

Is this feature request related to a problem?

For now we don't have any graphical visualization of Diagrams (not even a TUI), making tasks like example exploration or debugging tricky and slow.

Proposed solution

Use Gephi to dynamically visualize diagrams.
Preferably implement a serialization/deserialization of diagrams in C++.

Alternative solutions

Use other graph visualization libraries, like Graphviz' DOT format.

Additional context

This has been a long-awaited feature. A good thing in the long term would be to implement a serializer to/from TikzCD format.