Give a comprehensive config.json example in the docs

[joanise]

For someone who does not already know how to write json, it's not intuitive how to combine the different things we say you can add to our config.json file, by just reading the documentation at https://readalong-studio.readthedocs.io/en/latest/cli-guide.html#adding-titles-images-and-do-not-align-segments-via-the-config-json-file

There is already such an example, we should just make it more complete by also adding title, header and subheader to it, since that and images are going to be commonly combined.

And, there's a typo:

            "0": "image-for-page1.jpg",
            "1": "image-for-page1.jpg",
            "2": "image-for-page2.jpg",
            "3": "image-for-page3.jpg"

should be

            "0": "image-for-page1.jpg",
            "1": "image-for-page2.jpg",
            "2": "image-for-page3.jpg",
            "3": "image-for-page4.jpg"