Google Cloud Storage working with TechDocs (publish and fetch)

Plus
1. Introduce ncessary configs to connect with storage
2. Introduce config to prefer CI build of docs, or techdocs-backend to build and publish them.
3. Write guide how to use Google Cloud Storage with TechDocs
4. Add a TechDocs Configuration reference page in docucmentation

packages/techdocs-common/package.json

@@ -39,14 +39,16 @@
     "@backstage/backend-common": "^0.3.2",
     "@backstage/catalog-model": "^0.3.1",
     "@backstage/config": "^0.1.1",
+    "@google-cloud/storage": "^5.6.0",
     "@types/dockerode": "^2.5.34",
-    "@types/express-serve-static-core": "^4.17.14",
+    "@types/klaw": "^3.0.1",
     "cross-fetch": "^3.0.6",
     "dockerode": "^3.2.1",
     "express": "^4.17.1",
     "fs-extra": "^9.0.1",
     "git-url-parse": "^11.4.0",
     "js-yaml": "^3.14.0",
+    "klaw": "^3.0.0",
     "mock-fs": "^4.13.0",
     "nodegit": "^0.27.0",
     "winston": "^3.2.1"