docs(stream): add documentation for downstream operations

* Explain the concept and usage of downstream collectors in Java Streams

Author: ZahidFKhan

src/test/java/com/github/streams/learn/concepts/L What is a downstream?.md

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+# What is a downstream operation in Java Streams?
+
+In Java Streams, **downstream** usually means a **collector passed inside another collector**.
+
+You commonly see this in:
+- `Collectors.groupingBy(...)`
+- `Collectors.partitioningBy(...)`
+- `Collectors.teeing(...)`
+
+The outer collector decides the buckets (for example, by department or by pass/fail), and the downstream collector decides **what to do with each bucket**.
+
+## Why is it used?
+
+Downstream collectors help you:
+- aggregate grouped data (`counting`, `summingInt`, `averagingDouble`)
+- transform values while grouping (`mapping`)
+- post-process final grouped results (`collectingAndThen`)
+- build nested summaries (for example, group then average)
+
+Without downstream collectors, you often need extra loops or multiple passes.
+
+## Simple examples
+
+### 1) Count people per department
+
+```java
+var countByDept = employees.stream()
+		.collect(Collectors.groupingBy(Employee::department, Collectors.counting()));
+```
+
+Here:
+- outer collector: `groupingBy(Employee::department)`
+- downstream collector: `counting()`
+
+### 2) Collect employee names per department
+
+```java
+var namesByDept = employees.stream()
+		.collect(Collectors.groupingBy(
+				Employee::department,
+				Collectors.mapping(Employee::name, Collectors.toList())
+		));
+```
+
+Here the downstream `mapping(...)` transforms each `Employee` into only a name before collecting.
+
+### 3) Average salary by department
+
+```java
+var avgSalaryByDept = employees.stream()
+		.collect(Collectors.groupingBy(
+				Employee::department,
+				Collectors.averagingDouble(Employee::salary)
+		));
+```
+
+## Common downstream collectors
+
+- `counting()`
+- `summingInt / summingLong / summingDouble`
+- `averagingInt / averagingLong / averagingDouble`
+- `maxBy / minBy`
+- `mapping(...)`
+- `filtering(...)` (Java 9+)
+- `flatMapping(...)` (Java 9+)
+- `collectingAndThen(...)`
+- nested `groupingBy(...)`
+
+## Difference from stream intermediate operations
+
+- Intermediate operations like `map`, `filter`, `sorted` run on the stream pipeline.
+- Downstream collectors run **inside `collect(...)`**, typically per group/partition.
+
+## Tips
+
+- Prefer clear, readable collector chains over deeply nested one-liners.
+- Use `toList()` where appropriate in project code; use `Collectors.toList()` when required by APIs like `mapping(...)`.
+- For `maxBy/minBy`, remember values are `Optional`.
+