Replace all graphviz diagrams with raster images

[lparisc]

The current docsgen tool doesn't support Graphviz diagrams. The long term solution is to port all Graphviz diagrams to Mermaid, which is supported by our docsgen tool via a workaround. Official support might be added later.

In the meantime, we need to replace all Graphviz diagrams with their corresponding raster image. This affects mainly the Protocol Guides, although there are other docs that use Graphviz diagrams, including system diagrams. Replace those.

[lparisc]

Solution steps:

1. Obtain original graphviz/dot script.
2. Display in a graphviz/dot viewer (script contains one)
3. Export to .svg or .pgn raster image.
4. Replace script with raster image reference.