[DMN Designer] Learn DMN in 15 minutes

Finish the first stable version of learn-dmn-in-15-minutes.com.

Goals:

• Finish the content in all sections.

Acceptance criteria:

• As a user, I want to be able to read and interact with the FEEL section (finished ✅)
• As a user, I want to be able to read and interact with the Execution (finished ✅)
• As a user, I want to be able to read and interact with the Data Types section (finished ✅)
• As a user, I want to be able to read and interact with the Decision logic section (finished ✅)
• As a user, I want to be able to read and interact with the Keep learning section (finished ✅)