Add withApiKey() method and rewrite package README

Restructure documentation to follow elegant conventions:
- Add comprehensive table of contents with anchor links
- Write clear introduction with value proposition
- Use progressive complexity structure
- Apply consistent, polished writing style with imperative tone
- Add NOTE callouts for supplementary information
- Organize content into logical, navigable sections

* Add withApiKey() method for multi-tenant applications

Enable dynamic API key switching for multi-tenant scenarios:
- Add withApiKey() method that returns a new Pocket instance
- Store configuration internally for instance cloning
- Preserve all config options when creating new instances
- Add comprehensive tests for multi-tenant functionality
- Document usage in README with practical examples

Author: yannelli