Add Community Health files to EMF.cloud repositories

[Nina Doschek]

Add missing community documents to to all EMF.cloud repositories (where applicable):

• Code of conduct - CODE_OF_CONDUCT.md
  • Template: https://raw.githubusercontent.com/eclipse/.github/master/CODE_OF_CONDUCT.md
• Contribution guidelines - CONTRIBUTING.md
  • Example: https://github.com/eclipse-glsp/glsp/blob/master/CONTRIBUTING.md
• Security policy - SECURITY.md
  • Example: https://www.eclipse.org/security/policy.php, https://github.com/eclipse/rdf4j/blob/main/SECURITY.md
• Update README.md in case there are overlapping sections

Repos:

• emfcloud - #196
• emfcloud-modelserver - eclipse-emfcloud/emfcloud-modelserver#267
• modelserver-node - eclipse-emfcloud/modelserver-node#68
• emfcloud-modelserver-theia - eclipse-emfcloud/emfcloud-modelserver-theia#137
• model-validation - eclipse-emfcloud/model-validation#10
• coffee-editor - eclipse-emfcloud/coffee-editor#496
• ecore-glsp Remark: currently not planned to maintain this repo, hence no update of health files atm
• theia-tree-editor - eclipse-emfcloud/theia-tree-editor#71
• json-forms-property-view - eclipse-emfcloud/jsonforms-property-view#43
• modelserver-glsp-integration - eclipse-emfcloud/modelserver-glsp-integration#46
• emfjson-jackson - eclipse-emfcloud/emfjson-jackson#62