[Core] External docs fallback for unresolved crefs

[jeffpatton1971]

Summary
Add IExternalSymbolResolver + link policy. When cref can't be resolved internally, optionally link to external docs (e.g., learn.microsoft.com) instead of plain text.

Tasks

• Define IExternalSymbolResolver.
• Extend ILinkResolver to leverage resolver based on policy.
• Option: LinkPolicy.PreferExternalForUnknown + provider.

Acceptance

• When on: unresolved crefs link externally; when off: status quo.