Allow keyboard access to entire NavigationView by moving it down

[krschau]

Summary of the pull request

The NavigationView was badly interacting with the custom title bar, and it was impossible to tab to the top option ("Dashboard"). We solve the problem by moving the whole NavigationView down, under the title bar.

References and relevant issues

https://task.ms/46742514

Detailed description of the pull request / Additional comments

• Since we're moving all page content down, NavigationViewContentMargin doesn't need to be as big. New thickness aligns the header with the hamburger menu.
• Use a grid to move the page underneath the Title Bar
• Since we're no longer putting NavView content (hamburger icon) in the title bar, set IsTitleBarAutoPaddingEnabled="True". See https://learn.microsoft.com/en-us/windows/apps/design/controls/navigationview#top-whitespace
• Move title bar content left where hamburger was, so no need to set a margin in NavigationViewControl_DisplayModeChanged. This may need to get put back when we implement the back button ([Dev Home]Dev Home does not seem to make use of NavigationView's back button, making navigation around the app inconsistent #551)

Validation steps performed

PR checklist

• Closes Unable to access "Dashboard" navigation menu option using keyboard #1803
• Tests added/passed
• Documentation updated