Improve support for additional reference links

[EWSoftware]

Improvements need to be made in order to support links to additional reference content whether it is just to suppress the missing link warnings or to actually create links to external documentation sites. Related work items covered by this change (see the other work items for more details):

• Entity References: No inherited / additional reference linked entities #162
• BuildAssembler : warning : ResolveReferenceLinksComponent: Unknown reference link target [Web reference] #585
• qualifyHint of codeEntityReference not right working on unresolved references #614
• Microsoft Docs cross reference service cannot find all content (Member ID URL not found for target) #744
• How to reference dll and xml files from nuget package as a documentation source #769