docs: add plain-language ELI5 overview and link from README

Adds docs/eli5.md — a jargon-free explanation of Brainy and Cortex
using everyday analogies (smart librarian, turbocharger). Targets
non-technical readers who want to understand the project before diving
into the API. Links added at the top of README and in the Documentation
section index.

README.md

@@ -14,6 +14,8 @@
 
 Built because we were tired of stitching together Pinecone + Neo4j + MongoDB and spending weeks on configuration before writing a single line of business logic. Brainy unifies vector search, graph traversal, and metadata filtering so you don't have to choose.
 
+**New here?** → **[What is Brainy? — plain-language overview, no jargon](docs/eli5.md)**
+
 ---
 
 ## Install
@@ -297,6 +299,10 @@ Performance benchmarks and capacity planning in **[docs/PERFORMANCE.md](docs/PER
 
 ## Documentation
 
+### Start Here
+
+- **[Brainy explained simply](docs/eli5.md)** — Plain-language overview, no jargon, no code
+
 ### Core
 
 - **[API Reference](docs/api/README.md)** — Every method with parameters, returns, and examples