docs(no-release): Add guide for adding new APIs and issue templates

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This merge request adds documentation and templates to help contributors work with a GitLab Go client library. It creates two issue templates - one for reporting bugs and another for requesting new API features - that guide users to provide the right information when submitting issues. The main addition is a comprehensive guide that teaches developers how to add support for new GitLab APIs to the client library, using the branches API as a step-by-step example. The guide covers creating API files, implementing different types of endpoints (get, list, create/update, delete), setting up proper data structures, and integrating everything into the main client. This makes it much easier for new contributors to understand the codebase patterns and add new functionality consistently.