Better document Nginx dynamic modules

[FooBarWidget]

Nginx now supports dynamic modules. While we have instructions on how to compile Passenger as a dynamic module, a lot of places in our documentation (e.g. the RubyGems install page) still assume that Nginx only supports static modules. We should change that.

Furthermore, /install/nginx/install_as_nginx_module.html should provide an overall better explanation. Some people will get confused by the difference and relationship between APT, YUM, passenger-install-nginx-module and the normal Nginx module compilation process.