refactor: update SKILL.md for improved clarity and organization

- Renamed sections for better readability, changing "Tool usage" to "Instructions" and "Global flags" to "Authentication".
- Consolidated notes regarding `WEBFLOW_SITE_API_TOKEN` and `.env` file handling.
- Enhanced the description of `cloud.framework` resolution logic and command usage.
- Clarified flags and their requirements for `cloud init` and `cloud deploy` commands.
- Removed redundant information and streamlined the document for easier navigation.

Author: jallegretti-webflow