Feature/launch page

Still a WIP as I need some input on the content and need to fix a minor issue or two

WEBSITE-1082
This blurb was confusing and so I rewrote it: "The successors to MODs, Extensions are a new contribution type that require no file edits for installation. Extensions enable developers to develop new functionality for phpBB that is self-contained and void of actual core code changes. Board admins will appreciate the ease with which extensions can be installed, updated, and uninstalled via the ACP with just a few clicks. And the built-in version check functionality will help admins keep all their extensions up to date."