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

app-config.yaml

@@ -55,16 +55,15 @@ proxy:
 organization:
   name: My Company
 
+# Reference documentation http://backstage.io/docs/features/techdocs/configuration
 techdocs:
-  storageUrl: http://localhost:7000/api/techdocs/static/docs
   requestUrl: http://localhost:7000/api/techdocs
+  storageUrl: http://localhost:7000/api/techdocs/static/docs
+  docsBuilder: 'local' # Alternatives - 'ci'
   generators:
-    techdocs: 'docker'
+    techdocs: 'docker' # Alternatives - 'local'
   publisher:
-    # Ref: <Link to TechDocs publisher reference page>
-    type: 'local' # other options - google_gcs, aws_s3, azure_storage, etc.
-    # if type set to google_gcs
-    # google_token: 'abc'  # with write access
+    type: 'local' # Alternatives - 'google_gcs'. Read documentation for using alternatives.
 
 sentry:
   organization: my-company