doc(user/group): add docs for user & group crud

related to #CAM-957 #CAM-958
Merge branch 'master' of github.com:camunda/docs.camunda.org

Conflicts:
	site/src/documents/api-references/rest/single-page.html.eco
	site/src/documents/api-references/rest/toc.html.md
	site/src/files/app/js/pages.js