Add API.md (3200+ lines) with complete REST API documentation covering authentication, repository management, issues, pull requests, organizations, package registry, Actions, and Vault APIs. Includes code examples and error handling.
Add GUIDE.md with user-focused documentation for getting started, repository operations, collaboration workflows, and CI/CD setup.
Implement documentation tab in repository view with automatic detection and rendering of API.md and GUIDE.md files alongside README. Add locale strings and template for doc tab navigation.
