prepare extension for markdown use

During the conversion of https://codebase.helmholtz.cloud/hcdc/hereon-netcdf/hereon-netcdf-en/-/tree/convert-rst-to-md?ref_type=heads into markdown, some issues arose most likely connected to this extension:

• index.md: german umlaute vs authorlist hereon-netcdf-en#7 (closed)
• index.md: affiliation issue hereon-netcdf-en#9 (closed)
• summary_tables.md: .. metadata_list:: overloops hereon-netcdf-en#8 (closed)

All 3 issues have workarounds for the moments, but it would be great if we solve them with solid code. Would be great to have your expertise here @Chilipp .