Add multiprocessing examples and execution model definitions

Use multiprocessing to achieve true parallelism for CPU-bound task

Demonstrate GIL limitation using threading for CPU-bound task

rename 12_multiThreading_multiProcessing to 12_thread_concurrency_&_parallelism and remove sub folder 01_threads_concurrency

Learned basic Python threading:

Learned Python threading for concurrent I/O-bound tasks using requests.

Learn how to use Threading with example

Learned to compare threading and multiprocessing for CPU-bound tasks and GIL behavior.

Learned concurrency using threading vs multiprocessing in Python with examples

Learned File Handling with safe handling, creation and closing

feat: implement chai billing function with custom exceptions

Learned How to Create Custom Error Exception

Learned to create custom exceptions

Learned using custom exceptions

Learned using multiple exceptions.