Add comprehensive documentation for springboot-async-example

[sridharedu]

This commit introduces extensive documentation for the springboot-async-example module, aimed at users from beginner to advanced levels.

Key changes include:

1. Inline Comments in Java Source Files:

   • SpringbootAsyncApplication.java: Detailed explanations of Spring Boot setup, @EnableAsync, and custom thread pool configuration.
   • model/User.java: Comments on the DTO structure, including field explanations and Jackson annotations.
   • service/GitHubLookupService.java: In-depth commentary on the @Async service method, CompletableFuture, RestTemplate usage, logging, and error handling considerations. All comments are tailored for beginner, intermediate, and advanced understanding.

2. POM.xml Documentation:

   • Added comments explaining the structure of pom.xml, the role of spring-boot-starter-parent, individual dependencies, and the spring-boot-maven-plugin.

3. Application.properties Documentation:

   • Provided explanations on the purpose of application.properties.
   • Added commented-out examples and explanations for configuring the asynchronous task executor via properties, even though the project uses a programmatic approach.

4. Concepts.md File Creation:

   • Created a new concepts.md file in the springboot-async-example root directory.
   • This file provides a structured guide covering:
     • Introduction and high-level overview of the project.
     • Basic concepts (What is Spring Boot, Asynchronous Programming, @Async, CompletableFuture).
     • Intermediate concepts (Enabling async, thread management, TaskExecutor configuration, RestTemplate).
     • Advanced concepts (Detailed TaskExecutor customization, error handling, CompletableFuture composition, testing, and pitfalls).
   • Each concept is explained with definitions, analogies, code references from the project, and best practices.

This documentation aims to make the springboot-async-example module highly accessible and educational for developers at all skill levels, fulfilling the requirements of the original issue for this specific module.