Add the delete flag to remove old docs in stage and prod (#940)

We want to remove files that are in the remote but not source directory,
so that we stop indexing old content.

Tested on dev, this is for stage and prod. 

Note: cleaning up dev took almost 2 hours of action run time, for both
stage and prod we may see some timeouts on deploy.

Docs:

https://awscli.amazonaws.com/v2/documentation/api/latest/reference/s3/sync.html

.github/workflows/deploy.yml

@@ -37,5 +37,5 @@ jobs:
       - name: Deploy
         run: |
           aws s3 sync ./build s3://docs-s3.dev-ngrok.com/docs --delete --acl public-read
-          aws s3 sync ./build s3://docs-s3.stage-ngrok.com/docs --acl public-read
-          aws s3 sync ./build s3://docs-s3.ngrok.com/docs --acl public-read
+          aws s3 sync ./build s3://docs-s3.stage-ngrok.com/docs --delete --acl public-read
+          aws s3 sync ./build s3://docs-s3.ngrok.com/docs --delete --acl public-read