Removed hard-to-understand sentence from documentation introduction

refs #3068
Remove "What it means to use VWF" section

refs #3068
Update nav bar on top of 404 page to match the main page

refs #3068
Remove "Additional Information" section from documentation.md

This section pointed to a wikipedia page about VWF that I think would confuse the typical user of VWF.
refs #3068
Add VWF onion architecture image w/ a short description

refs #3068
Remove reference to system API in "The Drivers" section of documentation

We are going to remove the system API until we are able to complete it.
refs #3068
Correct links in architecture.md to point to other section of same page

They were pointing to separate pages that do not now exist since the documentation is all one page.
refs #3068