Fixed APIs being ignored, bunch of other tweaks

Removed UnityEngine.Internal.ExcludeFromDocsAttribute from exclude filters. Surprisingly many APIs marked with [ExcludeFromDocs] actually have an API reference page (for example, UnityEngine.Object(Object)).
Made DocFX output easier to separate from the program output, added DocFX exit code check.
Configured YAML serializer to omit serializing null values, the output now matches the original xrefmap.yml structure line-to-line.
For some reason, only references with overloads were accounted for, which is plain wrong. As a result, less than third of the API was mapped.
Reworked testUrls so that invalid URLs are excluded from the final references list.
Fixed href generation for some properties and simple overloaded methods.
I was unable to run the project as-is. Adding docfx.json and filterConfig.yml to build output fixed it for me.
Minor other tweaks.

NormandErwan / DocFxForUnity