Add Storybook-style component showcase for graph nodes

[AmitoVrito]

Description

Create a visual showcase of available graph node types with interactive examples.

What to do

• Create docs/graph/node-gallery.md or custom React page
• Show each node type as a card: node function, agent_node, rag_node, conditional edge
• Visual representation + code example + use case
• Click to see in a full graph example

[AmitoVrito]

Thanks for your interest! I will assign it to you. Before you start, could you give the repo a ⭐ on the main repo (https://github.com/SynapseKit/SynapseKit)? It really helps with visibility for a new project. Looking forward to your PR!