Add support for external documentation urls

[rpallares]

Hi,

I generate a documentation for a quite simple record that declares a nullable type.

public record MyRecord(Guid id, string? name);

The generation work well but it generate some bad link for NullableAttribute.

It consider that atttribute is internal whereas it's provided by Microsoft.
Here is the link generated: [NullableAttribute](system/runtime/compilerservices/nullableattribute)

In a more general way, could it be possible to add some external documentation definition to manage external libraries, frameworkd like asp.net ?

This probably could be done using a key-value collection (multi valued option or configuration file) and consider them inside GetDocsLink

I believe the tool could provide some values by default for all major Microsoft namespaces, and users could enrich with it's own or any external library.

Great tool anyway 😃