You created a page within your Style Guide.
Later on, you notice that this page should be in another document in the upper navigation. Don`t worry. You can easily move it over with the "Move Page" feature.

I will give you an example:
I have a page "Buttons" and wanna move it over to the document called "UI Library". Start the Edit mode by pressing the pencil in the Power Bar or use the shortcut CMD + E. 

On the right side you have the three dots to open the page actions. Please choose "Move Page".

Now you can select the target document into which you want to move the entire page. 

If you have any questions, feel free to contact us.