Add configuration option to hide "UNTAGGED" category

Is your feature request related to a problem?

When generating API documentation using tags, the plugin always displays an "UNTAGGED" category for operations without tags. In some cases, I want to hide this category completely as it creates unnecessary clutter in the documentation.

Describe the solution you'd like

Please add a new configuration parameter showUntagged: boolean (defaulting to true) that allows users to control the visibility of the "UNTAGGED" category. When set to false, operations without tags would be hidden from the documentation.

Describe alternatives you've considered

I've considered filtering the OpenAPI spec before passing it to the plugin, but this requires additional preprocessing steps and doesn't integrate cleanly with the existing workflow.

Additional context

I'm working on documentation for a large API where most endpoints are properly tagged, but a few legacy endpoints remain untagged. I want to present a clean, organized documentation experience that focuses on the properly categorized endpoints while we work on tagging the remaining ones.

No file or test path is identified in the issue. Start by locating the plugin configuration handling and the rendering of the "UNTAGGED" category, then trace how operations without tags are included. Done means a showUntagged option defaults to true and, when false, excludes untagged operations from generated documentation.