The developer portal consists of automatically generated pages like APIs, Products, Applications, Issues and manually written content. Since it is based on a content management system you can create such content as necessary.
To see the list of all existing content pages click on Content from the Developer portal menu in the publisher portal.
Click on the "Welcome" page to edit what is displayed on the home page of the developer portal. Make the changes you would like, preview them if necessary, and then click on Publish Now to make them visible to everyone.
The home page uses a special layout which allows it to display a banner at the top. This banner is not editable from the Content section. To edit this banner click on Widgets from the Developer portal menu, then select Home page from the Current Layer drop-down and then open the Banner item under the Featured section. The contents of this widget are editable just like any other page.