Inject Markdown files from filesystem into operation description

[nandorholozsnyak]

Is your feature request related to a problem? Please describe.
Developers tend to write descriptions for their endpoints but sometimes they can be super long and to write this into the production code can be fine until it is not.

Most times we recommend to break the rest controller into interface and implementations pairs, the interface can have the OpenAPI documentation descriptors and the Spring based annotations, and the implementation can be pure Java code.

As most editors are having Markdown support it can be wise to inject the descriptions from Markdown files and do not write lengthy descriptions in the annontations. With Java's text blocks it can be handy but still not good to write, because of the style and indentations.

Describe the solution you'd like
Markdown files could be put into the resources folder and upon startup the app would read them and would inject it into the specific operation's description:

File name suggestion priority:

• Custom extension
• operationId.md - in case of ambiguous warning log and no injection
• ControllerClassName.method.md

Custom extension:

    @Operation(extensions = {
        @Extension(name = "x-extender-description", properties = {@ExtensionProperty(name = "source", value = "CustomFile.md"))
    }))