Fix/rumdl gi issues

[rpietzsch]

addressing rumdl linter issues:

docs/explore-and-author/graph-exploration/graph-insights/features/groups.md:27:3: [MD036] Emphasis used instead of a heading: 'Remark: If user-defined classes are present, a second histogram is shown for better overview'
docs/explore-and-author/graph-exploration/graph-insights/navigating-to-cmem.md:31:1: [MD036] Emphasis used instead of a heading: 'From the Canvas'
docs/explore-and-author/graph-exploration/graph-insights/navigating-to-cmem.md:38:1: [MD036] Emphasis used instead of a heading: 'From the Details Pane'

please consider changing in your docu sources.