support generate multi doc files

[lucklyt]

Is your feature request related to a problem? Please describe.
my service have hundreds of apis, the generated api doc have so many apis and hard to read, before using swaggo, the api doc we manual maintenance have three files which apis are grouped.

Describe the solution you'd like
it it possible to add a feature , for example , api description have a new field , named group , and generated api info are in diffrerent files based on their group.