Feat: Allow multiple `api_paths` to be specified

[dothomson]

Currently our tech docs are covering services which multiple apis on different domains.

We're only able to reference a single api file within the configuration which has resulted in us combining public and internal endpoint specifications into the same openapi file.

Luckily we haven't had a name conflict yet, but for a logical separation it would be nice to enhance the api_path configuration allowing multiple files to be referenced.