Add documentation for using PackIcon

The current Using icons page on the wiki is very old and outdated. The library supports using the PackIcon class for setting these icons in a much simpler manner.

• Create a new PackIcon page for the wiki. It should include both a XAML and C# example of how to set the icon using the Kind property.
• Add the new PackIcon page to the wiki sidebar
• Delete the old Using icons page, and remove reference to it from the wiki sidebar.

[Keboo]

@rgomez90 looking really good. Just a few small changes:

• On the side bar lets move it from the Miscellaneous section to the Controls section
• In the first code-behind example, lets also include the using MaterialDesignThemes.Wpf;
• Just to alleviate any confusion, lets fully qualify the Brushes.Red to be System.Windows.Media.Brushes.Red

[rgomez90]

Will do this 😃