Sirius GitHub Pages redesign

**Creating new pull request to replace closed one. Image paths now fixed, was issue with how GH Pages generates paths vs local.**

New front-end, largely based on designs from Austin Wells in UX (approved by Jon Moore) handed off at the beginning of the summer.

Takes full advantage of Jekyll, using header/footer/includes.
Design didn't contain Overview content, so I incorporated my own design for that into the single-page structure.
Refactoring of markup and styles will be done in future commits unless something egregious is found.
Additional enhancements can be done in future commits (i.e. additional pages or content, search, design tweaks, etc).

**You can view this live at https://laurengalanter.github.io/sirius/**