[docs] app-bar docs do not provide information about logo

[DRiFTy17]

Describe the bug: The docs here do not provide context about how to add a logo, and there is no supporting content either.

I assume the reason for this is because the app-bar previously had the logo built-in to the component, and when it was removed, we overlooked updating these docs and code snippets.

We just need to add some information about what the component provides out of the box, how to include an icon (with both <forge-icon> and <img>/<svg>), as well as update the code snippets to reflect the icons being used.