docs: Review and update new glab duo cli docs
Description
This merge request updates the documentation and help text for GitLab Duo CLI features to improve clarity and consistency. The main changes include:
- Changed "GitLab Duo CLI" to "GitLab Duo for the CLI" throughout the documentation to better reflect the product naming.
- Enhanced explanations of what the tool does, clarifying that it's an AI assistant that works directly from the terminal and connects to the GitLab Duo Agent Platform.
- Restructured the docs with clearer sections, including a dedicated "Prerequisites" section and improved configuration options formatting with proper punctuation.
- Cleaned up the command examples by removing verbose output descriptions and adding clear comment headers to make them easier to understand.
- Added more specific information about authentication handling, binary installation location, and XDG Base Directory Specification compliance.