chore(docs): Update adding API support guide for new coding patterns

This documentation update simplifies the guide for adding API support to the GitLab Go library. The main changes include:

Code Simplification: The examples now use a new streamlined approach with a do() helper function instead of the previous verbose manual implementation. This reduces boilerplate code significantly - what used to take 20+ lines of code now takes just 4-6 lines.

Documentation Cleanup: Removed redundant explanations about ID parsing and URL encoding since these are now handled automatically by the new helper functions. The examples are much cleaner and easier to follow.

Enhanced Testing Guidance: Added a completely new section on integration testing that explains how to write tests that run against a real GitLab instance using Docker. This provides better validation than the existing unit tests that only mock responses.

The overall effect is that developers can now add new API endpoints with much less code while having better testing capabilities. The guide is more concise and focuses on the essential steps rather than implementation details that are now automated.