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.

Start by reviewing the existing concept structure, then compare concepts/streams/about.md, introduction.md, config.json, and links.json with nearby Java concepts. Cover stream creation, filter/map/reduce/collect, lazy evaluation, pipeline structure, and the stated use cases across these files. Done means the four Streams concept files are complete and consistent with the track’s conventions.

Written by the indexing model from the issue text.