Move blog articles into developer documentation

[mnapoli]

• [ ] How to create a custom theme in Piwik
• [x] How to create a scheduled task
• [x] How to create a widget
• [x] How to add new pages and menu items to Piwik
• [x] How to make your plugin configurable: merged into Plugin Settings
• [x] How to publish your plugin or theme on the Piwik Marketplace
• [ ] How to create a command
• [ ] Generating test data
• [x] How to make your plugin multilingual
• [x] How to write UI tests for your plugin
• [x] How to verify user permissions
• [x] How to write unit tests
• [ ] How to expose new API methods (published soon)

[mattab]

I'm bit concerned about having duplicate guides in the Blog and developer.piwik.org - what do you suggest we do to avoid duplicated guides?

[mnapoli]

After discussion, a solution would be to add a warning at the top of the blog posts to redirect visitors to the documentation.