NIFI-12566 Redesigned Project Website

- Redesigned home page and feature summary
- Restructured and simplified primary navigation
- Consolidated Downloads to single page with project sections
- Replaced Foundation and jQuery with UIkit framework

This closes #82

Co-authored-by: James Mingardi-Elliott <james-elliott@users.noreply.github.com>
Co-authored-by: David Handermann <exceptionfactory@apache.org>