/usr/share/doc/libsbml5/csharp-api is in libsbml5-cil-doc 5.10.0+dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libsbml5/../libsbml5-cil-doc/csharp-api
/usr/share/doc/libsbml5/../libsbml5-cil-doc/csharp-api