Improve docs feed

@parkr: let me know if you need me to make any changes.

I personally find it useful to have the `generator` property, but it's just cosmetic.

Also for the feed image, Firefox shows it right, Chrome doesn't. Keep in mind that these must be in `gh-pages` for the image link to work :)

I used https://github.com/jekyll/jekyll-logo/blob/master/jekyll-logo-light-transparent.png and resized it to 144x73 since the specs say that the maximum image width is 144px.