API documentation generation using Jazzy

### New Issue Checklist

- [x] I have tried with the latest version of OHHTTPStubs
- [x] I have read the [README](https://github.com/AliSoftware/OHHTTPStubs/blob/master/README.md)
- [x] I have read the [Using the right Swift Version of `OHHTTPStubs` for your project](https://github.com/AliSoftware/OHHTTPStubs#using-the-right-swift-version-of-ohhttpstubs-for-your-project) section
- [x] I have searched in the [existing issues](https://github.com/AliSoftware/OHHTTPStubs/issues?utf8=✓&q=is%3Aissue)
- [x] I have read [the OHHTTPStubs wiki](https://github.com/AliSoftware/OHHTTPStubs/wiki) to see if there wasn't a detailed page talking about my issue

### Issue Description

I am wondering if it would be nice to add [`jazzy`](https://github.com/realm/jazzy) to generate the API documentation.
This would:
- **expose the code documentation automatically**, reducing the workload of maintaining the README and the wiki
- **add information about API documentation coverage**: we could then see if the API is fully documented