Configure Circle CI workflows to use global context

Instead of defining per-project environment variables (1),
using context (2) allows to define and use environment variables
globally in all projects.

References:
  (1) https://circleci.com/docs/2.0/env-vars
  (2) https://circleci.com/docs/2.0/contexts

GH-44

.circleci/config.yml

@@ -113,6 +113,9 @@ workflows:
   version: 2.1
   build-multi-version:
     jobs:
-      - nodejs-v8
-      - nodejs-v10
-      - nodejs-latest
+      - nodejs-v8:
+          context: nord-docs-ctx
+      - nodejs-v10:
+          context: nord-docs-ctx
+      - nodejs-latest:
+          context: nord-docs-ctx