Add documentation for combobox

There are several additional features provided by the combo box that are not that easy to discover and should be documented.

• Add a page to the Wiki for ComboBox
• Add a link to it in the sidebar under Controls
• Document the two possible styles for ComboBox (MaterialDesignComboBox - this is the default, and MaterialDesignFloatingHintComboBox)
• Document that in addition to the default brushes on ComboBox there are a couple attached properties for changing colors. These are TextFieldAssist.UnderlineBrush for changing the color of the underline when the ComboBox is editable and the user is editing. HintAssist.Foreground for changing the color of the hint (when it is used).
• Document the ComboBoxAssist.ClassicMode attached property. When this is set to true it will cause the popup with the items to display strictly outside of the ComboBox rather than around it. This is more closely aligned with the default WPF ComboBox.