Surprisingly, there is no button or link that enables you to create a new page directly. Instead you have to follow the following (simple) procedure:

First you have to create a link to a page which does not exist already. Of course, you should place this link into a page that wants to refer to the new page (maybe because it is an overview page). You then click on the link you just created and are presented an information page about a missing page. As the information page suggests you choose the link “Create new page” to open a text input box which lets you comfortably enter the initial content of the new page. You may save, preview or cancel your content changes at any time.

To create a link to a new page in the current section just type:

[[documentation:writer_manual:name_of_the_page]]

To create a link to a new page in a certain section type :

[[:category1:category2:name_of_the_page]]

where, for example, you could choose :documentation:user_manual instead of :category1:category2.