Better docs

The goal is that the demo app should be copied into the gh_pages branch and become the actual documentation for ember-table. That will make it easier to contribute to the docs (everything is in the same repository) and will make sure the online docs are kept up to date.

Changes:
* Add community examples page
* Move migration guide into separate page
* Clean up footer
* Fix sidebar covering footer bug
* Add actual links to other open-source projects
* Other minor tweaks

@bigsley @charlescharles @dylnuge