Relays pages

[erskingardner]

• [x] Add a page talking about what Relays are and how they work
• [x] Add page with a list of different relays we know about (organized by language they're written in)

[erskingardner]

Related PR: #21

[erskingardner]

• [x] Create images showing how the network (clients, relays) functions
• [ ] Create images specifically for relay internals
• [ ] Link to NIPs specific to relays