Adding new user and user/repo shortcutst to Go form.
Starting to improve the shortcuts docs.
Main page layout and shortcut table.
Cleaning up shortcut table.
NbViewer
Addressing review comments:

* Center section headings.
* Use lowercase nbviewer for branding
* Eliminate shortcut table and replace by placeholder
* Adjust vertical spacing.
Merge pull request #239 from ellisonbg/goform

Clean up layout of main page and add shortcut table