In order to create a wiki sub page, you need to create a link to it first. Use the wiki page notation (e.g., [[My subpage]]) to link to a page in an existing one. Then, after saving the page, the link text will appear in red. The new wiki sub page can then be created by clicking on the red link.
Should you wish to create a hierarchy of wiki pages, you need to define parent-child relationships as follows:
In a child wiki page, click Rename.
In the Parent page field, select the page’s parent.
Click Rename.
To view the parent-child relationship, click Index by Title in the wiki sidebar.