Fix accessibility issues in What's New page

[krschau]

Summary of the pull request

Fixes four accessibility problem areas and one style bug:

• With text scaled up or a narrow window, the small card text (above the buttons) was getting cut off. Adding a ScrollViewer makes it readable. (I tried making the card longer and had trouble)
• With text scaled up or a narrow window, the big card text was getting cut off. Change the Width to MaxWidth=400 so that the text can wrap appropriately with the container.
• Text gets cut off in the Welcome to Dev Home Page  #1512 With text scaled up or a narrow window, the text at the top of the page was getting cut off. Changing from a Grid with fixed sizes to a StackPanel with a min size allows the area to size up with the text.
  • Also deleted junk "Text" from WhatsNewPage_Description
• Narrator reads "Link" for every FRE page card, even when there is no visible link #1722 Most cards do not have content in their HyperlinkButtons, but those HyperlinkButtons were still tab stops that could take focus. Change to only make them a tab stop if there is content.
• (Not accessibility) "Get started" button at the top of the page was cut off when the window is at its narrowest. Making the button slightly narrow prevents it from being cut off, but is still plenty wide enough to show text.

References and relevant issues

http://task.ms/46725464
#1722
#1755
#1512

Detailed description of the pull request / Additional comments

Validation steps performed

PR checklist

• Closes Narrator reads "Link" for every FRE page card, even when there is no visible link #1722
• Closes Text gets cut off in the Welcome to Dev Home Page  #1512
• Tests added/passed
• Documentation updated