feat(concepts): add Streams concept

### 📘 Description

This issue proposes the addition of a new **Streams** concept to the Java track on Exercism.

#### 🔹 Concept Overview
Java Streams offer a functional and declarative way to process collections. This concept introduces learners to:
- Stream creation from collections
- Common operations like `filter`, `map`, `reduce`, and `collect`
- Lazy evaluation and pipeline structure
- Real-world use cases for transforming and aggregating data

#### 📁 Files Added
- `concepts/streams/about.md`
- `concepts/streams/introduction.md`
- `concepts/streams/config.json`
- `concepts/streams/links.json`

This concept is designed to be beginner-friendly while introducing powerful tools for writing expressive Java code.