create stub documentation for unfinished modules on doc.silverstripe.com

[smagnusson]

To prevent the lakshans of the world from doing events modules without knowing they exist, create stub documentation for other modules on doc.silverstripe.com, e.g. for events module and subsites

to say something like "under development, only useful to developers who want to help and contribute to the module, look at <URL> to see list of open issues assigned there, and contact <module owner> for more information.

Then add to the main doc page under some sort of section like 'modules under development', etc.