feat: subgraph documentation

[razgraf]

This PR contributes content for the Subgraphs API section. The documentation for this part goes over

• the architecture (explaining entities and their purpose, important attributes like the parties involved in a stream)
• the official endpoints
• some sample queries (showcasing the most important filters and GraphQL features we use)
• a development section with a few details about the recommended setup and a short project bootstrap tutorial

Fixes #7.