I'm building documentation for a plugin I have written, for a third-party application, that has its own API that I use in my plugin. The API is documented elsewhere (website) and I know how to generate urls for each of the API types.
During the docs build of my plugin, SandCastle naturally complains about the missing third-party API types since they are not defined in my plugin assembly and are part of the host application API assembly that I have referenced in my plugin project.
I'd like to know how I can configure SandCastle to resolve the missing types to the correct url and include that in the generated .chm or website files as hyperlinks.
SandCastle already does that for all dotnet types and routes them to MSDN documentation correctly. SandCastle seems to have built-in url resolvers for these known sources internally but I don't see any way of extending that, adding my own resolver.
An exact answer is difficult for application documentation, since a large dependence on your needs and environment is given (Visual Studio Plug-In, SHFB stand-alone).
You know - Sandcastle was designed to document code based on a set of XML comments and API information found in a reflection information file. As such, it requires at least one assembly in order to produce a help file containing API documentation. If you do not include at least one assembly with a namespace containing at least one public class, the help file cannot be produced.
I'm not quite sure it meets your requirements; you should check my several years old answer in another context (Documenting external dll's with sandcastle).
However and as described as a second solution (above link), you can use the Additional Content Only Plug-In to produce a help file based on conceptual content alone. To do so, add the plug-in to the project via the Plug-Ins project property category, add conceptual content to the project, and build the help file. See the Conceptual Content topic for more information about how to utilize conceptual content.
You may want open a small example.
FYI - for further information search SO by tag e.g. https://stackoverflow.com/questions/tagged/sandcastle