Updating the SourceForge landing page files

* Added a "How to Get Involved" topic
* Added "Get Involved" to the left navbar
* Added a link to the "How to Get Involved" topic from the "Who We Are" topic
* Modified the link-lists on the "About" topic. The "Getting Started" item now
points to the documentation; the "Read the Docs" item is commented out, and I
added a "Who We Are" item

Jarno, you'll need to add processing for the new "Who We Are" item on the landing page; it has an outputclass of "weweare"