API references starting with path /grpc/ aren't shown as external links

[chalin]

API references starting with path /grpc/ aren't shown as external links (this is inconsistent with how we show /api/ path links). For example, see Asynchronous-API tutorial > Overview:

Also, as another consequence, these "external" (after the rewrite rules are applied) links don't open up in another view.

Releated #414