Replicate WinUI3 Gallery's design guidance sections and rebuild in JS as appropriate

[chrisglein]

From #405

The design guidance section of WinUI 3's gallery is super interesting. It's a great place to get the brush names for building pages and controls. It's very Fluent, by design. Having coverage for how to navigate that as an RNW app would be helpful.

Accessibility

The most useful section would be Accessibility. It covers screen readers, keyboard support, and has a color contrast checker (which is redundant if we just point folks to Accessibility Insights). "How do I make my RNW accessible?" is a topic we can and should cover in Gallery.

Typography

This page could be replicated... but we don't have the Style types that folks can copy. It'd just be design guidance. Unless we incorporate #425 and it has styles we can guide people with.

Iconography

There isn't an intrinsic FontIcon type like there is in XAML. But you can get good usage out of the icon font without having that simple wrapper. This page would more be a glyph finder, in app instead of here: https://learn.microsoft.com/en-us/windows/apps/design/style/segoe-ui-symbol-font

Colors

This is a very useful page for finding brush colors, because XAML's brush naming patterns are not always obvious. What I think it means for an RNW app is "How do I effectively use PlatformColor?"

[chrisglein]

At least for the colors part, I've started cooking up something for that: https://github.com/chrisglein/react-native-gallery/tree/platformColorPage
Particularly as we navigate the Paper->Fabric transition, it's helpful to clarify what PlatformColor values are valid.