docs: clean up docs for azp migration (#13558)

Signed-off-by: Lizan Zhou <lizan@tetrate.io>

Mirrored from https://github.com/envoyproxy/envoy @ e136b95a6083aa9b284bb7acfe704a259c7ffa0c

CONTRIBUTING.md

@@ -50,11 +50,11 @@ generated RST files are also viewable in `generated/rst`.
 
 Note also that the generated documentation can be viewed in CI:
 
-1. Open docs job in CircleCI.
+1. Open docs job in Azure Pipelines.
-2. Navigate to "artifacts" tab.
+2. Navigate to "Upload Docs to GCS" log.
-3. Expand files and click on `index.html`.
+3. Click on the link there.
 
-If you do not see an artifacts tab this is a bug in CircleCI. Try logging out and logging back in.
+If you do not see "Upload Docs to GCS" or it is failing, that means the docs are not built correctly.
 
 ### Documentation guidelines