Reorganize the documentation for better accesibility

The Mailman's documentation on RTD is quite awesome, but I find it very hard to navigate around because of so loooong list on the left which makes it quite difficult to find anything even if I know the path. This makes it a little easy by grouping the documentation by modules.

Also, I have re-organized some of the pages on the top so that the ones that "users" would first want to look at is at the top and the contribution help follows.