Add Documentation for Custom Handlers

[SakshayMahna]

Goal

LaunchMap supports custom ROS 2 handlers (example, for utilities like RewrittenYaml and LaunchConfigAsBool). However, new contributors and users currently have no clear guide on how to write and register their own handlers.

This issue aims to create documentation that walks through the process of building a simple custom handler, so others can extend LaunchMap easily.

Acceptance Criteria

• Add a new section to the documentation (example, docs/custom-handlers.md).
• Include:
  • Explanation of what a handler is.
  • How to register custom handler directory through VSCode.
  • The @register_handler decorator and its usage.
  • Example: Implementing a handler for LaunchConfigAsBool (refer to parser/tests/real_cases/launch_files/nav2_bringup.launch.py)

Reach out for more information / clarification.