Mention video posts in the introduction article

go/blog/blog.yaml

    * `link`, with an additional `url` field
    * `image`, where `url` is the source of an image
    * `song`, where `url` is the source of the song
    * `video`, where `url` is a link to a YouTube video
    With the exception of the `shell` type, `title` and `content`
    are optional.