update doc of serialization

[micjoswig]

Our serialization is documented here. To me it seems that the list of currently supported types is incomplete; please update.

@ThomasBreuer please add a section on groups

@JohnAAbbott in the long run, it might make sense to also mention CoCoA here

[fingolfin]

That list contains at least one entry that is not a type polynomial_ring.

We also need documentation for the actual serialization format. Currently as far as I know nothing exists, so anyone who would like to provide a separate implementation of the format essentially needs to reverse engineer it.

[JohnAAbbott]

I am discussing the details with @antonydellavecchia We hope to produce something concrete fairly soon (at least a first draft). I shall also discuss with @JHanselman this afternoon

[ThomasBreuer]

I had been thinking of a function is_serializable(obj) that checks whether a serialization method is available for obj.

I think the approach from #3768 is right: List prominent examples of serializable types in the documentation, and provide functions in Oscar that give detailed information based on the actual code, as in reverse_type_map.

Perhaps we can then add a rule whether element types of serializable types shall be listed as serializable in the documentation, or whether one can assume that an element of a serializable object is serializable.