docs(developer): restructure the Crowdin Apps section and add the SDK… - #907

… article

Adds an article about the Crowdin Apps SDK and regroups the Crowdin Apps section around it.

• New Crowdin Apps SDK article: what the framework handles for you, how to install it, its requirements, and where apps built with it deploy. The card on About Crowdin Apps now opens it instead of the SDK's own site.
• The Serverless Apps article is now Serverless Apps SDK and opens with the same kind of overview, so the two read as a pair. Its URL is unchanged.
• The App Descriptor, Crowdin Apps JS, and Security for Crowdin Apps articles moved into a new Reference group, so the section lists the articles you read start to finish first and the ones you look things up in last.
• The Publishing articles now follow the order an app author goes through: Releasing, Publishing Your App, Monetization, Promoting.
• Removed the User Interface article. It recommended picking a third-party CSS framework, while both SDKs now ship a UI kit styled to match Crowdin. Its URL and the older /developer/crowdin-apps-ui-kit/ URL redirect to the SDK article.
• Added a redirect from /developer/crowdin-apps-module-translation-alignment/, which external links point to, to the Translation Alignment Processing module article.