[REQ] Document configuration file reference format

Is your feature request related to a problem? Please describe.

We are using the maven plugin of the generator, however we feel like yaml is easier to read and modify compared to the pom.xml, so we want to make use of the config file you can pass to the generator. However I'm not able to find a full reference for the configuration file - I know some of the options depend on the generator, but stuff like templateDir or verbose should also be documented somewhere. Also in doc sections like these, there is a files key shown, but I can't find any documentation on the available options.

Describe the solution you'd like

I want a page in the docs that lists all available (global) config options for the config file, that are not covered by openapi-generator-cli config-help -g <generator>.

Describe alternatives you've considered

None

Additional context

None

Start by reviewing the configuration-file documentation and the customization page's user-defined templates example, then compare them with the options exposed by openapi-generator-cli config-help -g <generator> and the Maven plugin. Done means a documentation page lists all global config-file options not covered by generator-specific help, including options such as templateDir, verbose, and the files key.