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
"installed via the ACP" or "installed .. with just a few clicks"? Seems misleading, doesn't it also require using a 3rd party tool to upload the directories and files? Will the "built-in version check" work for externally sourced Extensions? 

To add those points: "The successors to MODs, Extensions are a new contribution type which require no file edits for installation. Extensions enable developers to add new functionality for phpBB that is self-contained and void of actual core code changes. This should greatly reduce problems when updating the software. Board admins will appreciate the ease with which extensions can be installed, updated, and uninstalled via the ACP with just a few clicks, once the files are uploaded. The built-in version check functionality, for extensions sourced from the Extensions Database, will help admins keep their extensions up to date."