Update Step 3.5 with full CYOA interface registration guide (#157)

* Update Step 3.5 with full CYOA interface registration guide

Replaces the outdated single-example CYOA section with a comprehensive
guide covering multiple scenarios (single uplink, L3/L2 port channel,
dual uplinks), including Mermaid diagrams, interface tables, and CLI
examples for each.

* Expand How It All Fits Together section and improve Step 3.4

* Address PR review comments from Ben

* Fix Mermaid node labels with parentheses in device type diagrams

Author: thijsvanemmerik