Add diagram guidelines to style.md

We use mermaid js for diagrams

Add conventions for diagrams to style.md, e.g.:
- Blockchains move to the right (for shorter chains) and down (for really long chains)

Open style.md and review the existing style guidance in the context of Mermaid JS diagrams. Add conventions for diagram layout, including the stated rightward direction for shorter blockchains and downward direction for very long chains. Done means the requested diagram guidelines are documented in style.md.

Written by the indexing model from the issue text.