Update WinUIGallery directory to be aligned with WinUI Templates

[karkarl]

Current WinUI-Gallery directory is disorganized and hard to navigate and maintain. The repo should be updated to be aligned with proper WinUI development standards. Examples below:

BlankTestApp

ContosoCamera

Namely, pages other than App.xaml and MainPage.xaml should not be in the root directory. Other page folders should be organized in more intuitive subdirectories.

[Jay-o-Way]

What does this mean? Does this touch #1246 and/or #1542 ?

[lostluna1]

I guess what the publisher of this theme is referring to is to make WinUIGallery consistent with the design patterns created by the WinUI extension

[karkarl]

@karkarl Please describe the idea for this issue

Updated the description. Yes, it is aligned with the issue you have opened. The current directory organization doesn't make too much sense! Feel free to take a stab at the issue :)