[Feature Request] Allow SourceLink support to be disabled

**Is your feature request related to a problem? Please describe.**

I have xrefmap YAML files for dependencies of my library, with appropriate base URLs. Before docfx version 2.64.0, this meant that the generated documentation for my library would link to the generated documentation for my dependencies (e.g. for parameter types, return types, "see" elements etc). Great.

Since docfx 2.64.0, the links to documentation have been replaced by links to source code, for any dependencies which are generated with SourceLink. This isn't nearly as friendly IMO, and I'd like it to at least be configurable.

**Describe the solution you'd like**

Either:

- Link to the documentation instead of the source if an xrefmap is provided
- Allow a preference to be configured between "link to source" or "link to documentation"

**Describe alternatives you've considered**

I've looked to see if this is already available, and I can't find anything. I've tried to work out how I could "intervene" (e.g. by deleting pdb files) but there's nothing within my project's directory that looks like it would be suitable to do this.

**Additional context**

I can provide a repro if desired - it'll just take a little while to create a minimal one.