Add architecture diagram and documentation

[manaraph]

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[github-actions]

The latest Buf updates on your PR. Results from workflow Go CI/CD / pipeline (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 2, 2026, 8:28 PM

[github-actions]

⬆️ Go test coverage increased from 28.2% to 85.5% compared to 4977cdd (6 ignored files)

Updated Package Coverages:

# Package Name                                                      |  Prior |    New
+ github.com/manaraph/stream-aggregator/internal/services/generator |   0.0% |  92.3%
+ github.com/manaraph/stream-aggregator/internal/services/ingestion |   0.0% |  80.8%
+ github.com/manaraph/stream-aggregator/pkg/broker                  |   0.0% |  94.1%
+ github.com/manaraph/stream-aggregator/pkg/grpcapi                 |   0.0% |  83.3%

View coverage for all packages

# Package Name                                                      | Coverage
+ github.com/manaraph/stream-aggregator/internal/services/gateway   |    86.7%
+ github.com/manaraph/stream-aggregator/internal/services/generator |    92.3%
+ github.com/manaraph/stream-aggregator/internal/services/ingestion |    80.8%
+ github.com/manaraph/stream-aggregator/pkg/broker                  |    94.1%
+ github.com/manaraph/stream-aggregator/pkg/grpcapi                 |    83.3%
+ github.com/manaraph/stream-aggregator/pkg/ws                      |    86.8%