fix: add reference/*.md to docfx content glob (#2248)

Without this, the reference subdirectory pages are not included
in the docfx build and return 404 on the published site.

Author: mnriem

docs/docfx.json

@@ -4,7 +4,8 @@
       {
         "files": [
           "*.md",
-          "toc.yml"
+          "toc.yml",
+          "reference/*.md"
         ]
       },
       {