docs: refresh Platform architecture diagram (#179)

Replace the radial dj-platform.png with an updated SVG showing the
open-source core (relational database + code repo + object store)
inside a DataJoint Platform frame, with surrounding services labeled
explicitly. Switching to SVG keeps the asset small (~10KB vs 160KB)
and scales cleanly at any zoom level.

Adapted for the conceptual data-pipelines page:
- AI bar labeled "AI Interface" (protocol-agnostic, was "(MCP)" in the
  source slide deck) — keeps the doc durable across protocol choices
- Bottom band reworded from governance/compliance language
  ("Governed Execution / Access Control / Audit Logging / Provenance")
  to generic pipeline qualities
  ("Reproducibility / Provenance / Integrity / Collaboration"),
  which fits a pipelines architecture overview rather than a Platform
  product pitch

Also lightly revise the surrounding prose to make the
open-source-core vs. Platform layering explicit, and relabel the
core-component table header to "Open-Source Core".

Delete the now-unused dj-platform.png.

Author: dimitri-yatsenko

src/explanation/data-pipelines.md

@@ -10,12 +10,12 @@ A **scientific data pipeline** extends beyond a database with computations. It i
 
 ## Pipeline Architecture
 
-A DataJoint pipeline integrates three core components:
+At the heart of every DataJoint pipeline is an **open-source core** of three components that handle schema, computation, and storage. The managed **DataJoint Platform** extends this core with services for AI access, data ingestion, exploration, security, collaboration, and visualization.
 
-![DataJoint Platform Architecture](../images/dj-platform.png/)
+![DataJoint Platform Architecture](../images/dj-platform.svg/)
 
-| Component | Purpose |
-|-----------|---------|
+| Open-Source Core | Purpose |
+|------------------|---------|
 | **Code Repository** | Version-controlled pipeline definitions, `make` methods, configuration |
 | **Relational Database** | System of record for metadata, relationships, and integrity enforcement |
 | **Object Store** | Scalable storage for large scientific data (images, recordings, signals) |