If you have write access to a wiki, you should be able to see the Add a custom sidebar and Add a custom footer buttons.
GitHub has a default sidebar where it places all the pages of the wiki. This might not be useful since they are shown in name order and sometimes you want users to be able to access important information without searching too much.
Much like any other wiki page, the sidebar can be written in a markup language that GitHub supports. In the following example, I used Markdown:
As you can see, I used a bulleted list and placed links on each item. Indenting an item (one or more spaces) ...
