Add more information to the Pages guide

• How to test a page (with UI tests). We only have to mention it in a section and can link to the guide
• We need to link to the Check permission guide and mention they have to take care of it if needed
• Explain what they can do with pages (pretty much anything) (refs #80)
• Explain how to place a page under the User section (extend user.twig in template)
• Explain how to create a blank page (no extends *.twig) at the beginning of the template
• Explain how to access $_GET and $_POST parameters via Common::getRequestVar
• Not sure if it is needed how to call API methods via Request::processRequest. Probably rather not, this would be a separate guide maybe under Utils named "API Requests"? Shall we create an issue for this?

Parts of it can be maybe used from http://developer.piwik/guides/controllers

Start with the Pages guide and compare its existing content with the controllers guide at developer.piwik/guides/controllers. Cover UI-test linking, permission checks, page placement and blank templates, and Common::getRequestVar; decide whether Request::processRequest belongs in a separate API Requests guide. Done means each requested topic is documented with the relevant links and examples.