[REQ][openapi-yaml] support merging multiple openapi YAML files into one using generator

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

No.

Describe the solution you'd like

I have multiple openapi YAML files. However the ReDoc tool I am using generates separate document per file. They don't link to each other.

In order to have a standalone doc generated, I was thinking merging all yaml files into one manually. However it increases cost of maintenance. Thus I am counting this openapi-yaml docgen to merge the YAML files automatically.

Describe alternatives you've considered

There are other tools I could use, but it means I have to introduce them into our CICD pipeline. As we are already using openapi-generator, I'm thinking why not have the generate to do the merge job as well, which is more elegant.

Additional context

Start by locating the openapi-yaml documentation generator and its handling of input YAML files. Determine how multiple specifications could be supplied and what merging rules are required, then define completion as producing one standalone document from the inputs without requiring manual maintenance.