documentation: each algorithm should list all its action functions, including C/Fortran bindings

[c-dilks]

Instead of one Doxygen \xrefitem, via the alias \action_function, let's create an \xrefitem for each algorithm. This will put all the links to an algorithm's action functions on a separate page. It also allows us to use the same \xrefitem in generated C/Fortran bindings, to make them more discoverable.