Extensions pages

Add or update the following Extensions page: Home, Extensions Team Overview, phpBB Official Extensions, Extensions Development Team Overview, Installing, Extensions Rules and Guidelines, Recognised Extension Developers.
Okay. I added the unit tests in https://github.com/battye/phpbb-website/commit/61a7e26070dbeaceeb0da67e0dafe5302b4fa0d7 

Do you want me to create a new pull request so these latest changes can get merged in?