You have a variety of ways to manage your pages as you edit them.
You can preview a page by clicking on the preview page icon next to the save button. It will open up the page in a new tab, and should work for public and hidden pages.
Click the gear next to the save button, then choose change page slug to edit the page URL. Don't forget to Save.
Clicking on the eye icon when you hover over the page, will allow you to hide the page from the public. When a page is hidden, only people with a direct link can view it.
There are multiple ways you can add a sub category to a specific page. The first method is to click on the + button on the page you want to nest it under. The second method would be to simply click and drag the page underneath slightly to the right.
There should be a button next to the category name that allows you to move categories between Documentation and Reference, as long as there are no API reference pages within the category.