docs: add SVG diagrams and Semantica wordmark logo

Diagrams (docs/assets/img/diagrams/):
- architecture-overview.svg: 4-column layered architecture
- pipeline-flow.svg: 8-step numbered pipeline flow
- kg-structure.svg: entity/relation graph with typed nodes and labeled edges
- graphrag-flow.svg: dual-path retrieval (vector + graph) to LLM to grounded answer
- extraction-pipeline.svg: NER/Relation/Coreference fan-out to Triplet Generator
- agent-context-flow.svg: AgentContext hub with VectorStore and ContextGraph
- reasoning-chain.svg: forward-chaining inference with explanation path

Wordmark logo (light + dark SVG variants):
- Green rounded-square S icon + Semantica text in green
- docs.json updated to use wordmark SVGs for light and dark modes

Pages updated with diagrams:
- index.md, architecture.md, quickstart.md, concepts.md
- reference/kg.md, reference/pipeline.md, reference/semantic_extract.md
- reference/context.md, reference/reasoning.md

docs/quickstart.md

@@ -39,6 +39,8 @@ python -c "import semantica; print(semantica.__version__)"
 
 ## Full Pipeline
 
+<img src="/assets/img/diagrams/pipeline-flow.svg" alt="Semantica end-to-end pipeline: Ingest → Parse → Normalize → Extract → Build KG → QA → Store → Deliver" style={{ width: '100%', borderRadius: '10px', margin: '0 0 24px' }} />
+
 <Steps>
 
 <Step title="Ingest">