Doc Alignment: Button Improvements

[stmoy]

Describe the bug

Several button (& related) improvements:

• Update ToggleSplitButton sample per the docs: https://docs.microsoft.com/en-us/windows/uwp/design/controls-and-patterns/buttons#differences-with-togglebutton
• Update Button sample showing graphical content to look like the doc example showing a photo button: https://docs.microsoft.com/en-us/windows/uwp/design/controls-and-patterns/buttons#button-content
• Add a sample to DropDownButton page with a RichEditBox including alignment to show a DropDownButton in situ: https://docs.microsoft.com/en-us/windows/uwp/design/controls-and-patterns/buttons#create-a-drop-down-button

[marcelwgn]

Should those all be new samples or should we change existing samples to align with the docs here?