Docs: using ShellIcons with ShellUI #60

Summary

ShellUI's built-in chrome icons (chevrons, close buttons) are inline SVG, and content icons go through Icon slots, so any icon library works. There's no page showing how to use ShellIcons there.

The page should cover

• Installing ShellIcons.Blazor and passing icons into Icon slots
• Never adding @using ShellIcons.Icons to _Imports.razor: icon type names (Calendar, Menu, …) collide with ShellUI components. Use the <ShellIcon Name="…" /> dispatcher or a namespace alias instead.