docs(concurrency): add explanation of why thread pooling is used in Java

WHAT was added:
- Detailed notes on the advantages of thread pooling in concurrent programming.
- Covered the following key benefits:
  1. Resource Management → avoids costly thread creation/destruction overhead.
  2. Faster Response Time → pre-initialized threads reduce task latency.
  3. Control Over Thread Count → prevents resource exhaustion by limiting threads.
  4. Avoids Thread Overhead → minimizes context switching, optimizes CPU utilization.
  5. High-Concurrency Use Cases → ideal for servers, DB connections, real-time apps.

WHY this matters:
- Provides clear justification for using thread pools instead of raw threads.
- Highlights performance, scalability, and stability improvements.
- Prepares foundation for deeper topics like ExecutorService, ScheduledExecutorService, and custom thread pools.

HINTS / NEXT STEPS:
- Add examples using `Executors.newFixedThreadPool()` and `newCachedThreadPool()`.
- Compare pooling strategies (fixed vs cached vs scheduled vs single-threaded).
- Show performance difference with and without pooling via a small benchmark.

KEYWORDS:
java threadpool concurrency executors performance scalability.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section19MultiThreading/ThreadPooling/src/ExecutorsFramework/Thread Pooling - Why Use It.txt renamed to Section19MultiThreading/ThreadPooling/src/Thread Pooling - Why Use It.txt

@@ -21,4 +21,4 @@ for different tasks.
 - Ensures optimal CPU utilization by reusing existing threads.  
 
 5. Useful in High-Concurrency Environments
-- Ideal for web servers, database connections, and real-time applications where multiple tasks need execution without delay.
+- Ideal for web servers, database connections, and real-time applications where multiple tasks need execution without delay.