Document `embed::ImageSource::attachment` usage

[laralove143]

To add a file image to an embed, you need to add an attachment to the message and call ImageSource::attachment with the same filename. The documentation could give information on this.

The example at EmbedBuilder::image also only shows how to use it with URLs, which is simpler to do. There could be an example for files as well.

Although you could make assumptions or check the Discord docs for this, I think these additions fit nice in Twilight documentation itself. I'm willing to PR this if the changes are approved.