Add Kite start page as index.html.

This isn't ready to merge yet, but I'd like to get feedback early. This moves the current docs index to all-pages.md and replaces it with a page to introduce the Kite project. That start page links directly to the CLI tutorial, the dataset guide, and the morphlines guide.

I've also updated the footer with links the JIRA, github, etc and added some style changes that I think help (should be viewed in jekyll serve).