docs: images have too much vertical padding

Very minor but annoying issue is that when rendered, the resulting images from the mermaid diagram code doesn't wrap tightly around the image parts and makes for a rather ugly bit of whitespace above and below the images.

https://github.com/canonical/workshop/blob/ea9b86d9ca13ae212887eaf5ff7541196f4ce634/docs/explanation/interfaces/tunnel-interface.rst?plain=1#L109