Solidus: Split API documentation in multiple files

[kennyadsl]

Follow-up to #3252.

Right now we have a single 6k-row file (openapi/api.oas2.yml) that describes our API, we should investigate if makes sense and there is a way to split this file into multiple smaller ones, in order to make changes easier.

[JuanCrg90]

I'll be working on this I hope to send a PR soon

CC @aldesantis @kennyadsl

[kennyadsl]

@JuanCrg90 please take a look at instructions on how to work/contribute to API as well: https://github.com/solidusio/solidus/tree/master/api#documentation. We changed that after Stoplight Studio has been released and I'm concerned that it's not compatible anymore with splitting this file into multiple ones, but I could be wrong. Let us know!

[kennyadsl]

@aldesantis and I tried to change API documentation recently and we think this is no more needed now that we have Stoplight Studio. Going to close, thanks anyway @JuanCrg90