Changed node labels from 'Node X (relay-list.txt)' to simply 'Relay X'
for cleaner visualization. Also updated Concepts section to use 'Relay'
terminology consistently throughout the documentation.
The relay list is now decentralized - each node maintains its own
local relay-list.txt file. Nodes share feeds directly with each other
in a P2P fashion, without needing a centralized list endpoint.
Changes:
- Remove 'List nodes' from mermaid diagram
- Add relay-list.txt notation to each node
- Update 'Share Users' to 'Share Feeds' for accuracy
- Simplify Concepts section to reflect decentralized architecture
