fix: enhance README and GitHub workflows for clarity and functionality

[soderlind]

This pull request introduces a major refactor and modernization of the WordPress GitHub plugin updater, focusing on a clearer API, improved documentation, and robust workflow automation. The most significant changes are the introduction of a canonical static API (GitHubUpdater::init), a streamlined and more informative README.md, enhanced error handling, and improved GitHub Actions workflows for building release assets.

Key changes include:

API Modernization & Code Refactor

• Introduced a new GitHubUpdater class with a static init method as the canonical API for plugin update registration, deferring initialization safely to the WordPress init hook and improving error handling and type safety. The legacy GitHub_Plugin_Updater is now a wrapper for backward compatibility. (class-github-plugin-updater.php)
• Refactored the legacy updater to delegate to the new static API, improved type casting for configuration parameters, and removed redundant setup logic. (class-github-plugin-updater.php)
• Added a guard to prevent direct file access by checking ABSPATH. (class-github-plugin-updater.php)

Documentation Overhaul

• Completely rewrote README.md for clarity: emphasizes the new static API, provides concise setup instructions, details requirements and troubleshooting, and documents workflow integration for release assets. Also clarifies backward compatibility and best practices. (README.md)

Workflow Improvements

• Updated the release workflow (on-release-add.zip.yml) to use a parameterized plugin zip name, optimized Composer install, and added a verification step to ensure the required dependency is included in the zip. (on-release-add.zip.yml)
• Added a new manual workflow (manually-build-zip.yml) for building and releasing plugin zips on demand for a specific tag, mirroring the improvements in the release workflow. (manually-build-zip.yml)