Add GitHub templates for the discussions forms

[ndoschek]

Similar to the Issue templates, GitHub just released discussion templates:

• https://github.blog/2023-01-09-github-discussions-just-got-better-with-category-forms/
• https://docs.github.com/en/discussions/managing-discussions-for-your-community/creating-discussion-category-forms

Suggestions to include to the emf.cloud discussion template:

• Currently, people are not using the "Mark as answer" to our liking, often we have to mark them as answered ourselves.
  We could add this as information text (as text comment) now that templates are available.