All developer documentation is bundled and located at https://developer.atlassian.com.
The Confluence module types: For more details about the available modules and which arguments and elements they accept, visit https://developer.atlassian.com/display/CONFDEV/Confluence+Plugin+Module+Types.
Confluence Plugin Tutorials: Various Confluence tutorials from beginner to advanced developer are available at https://developer.atlassian.com/display/CONFDEV/Confluence+Tutorials.
Atlassian Design Guidelines and User Interface: Atlassian provides a toolkit for you to help you create beautiful features for their products at https://developer.atlassian.com/display/AUI/.
The Marketplace: Everything you need to get your plugin Marketplace-ready is available at https://developer.atlassian.com...