Merge branch 'ci/migrate_doc_server_v4.0.0.0' into 'release/v4.0.0.0'

chore: Migrated doc preview server (v4.0.0.0)

See merge request application/esp-at!1630

Author: xcguang

.gitlab-ci.yml

@@ -578,7 +578,7 @@ deploy_docs_preview:
   variables:
     TYPE: "preview"
     DOCS_BUILD_DIR: "${CI_PROJECT_DIR}/docs/_build/"
-    DOCS_DEPLOY_PRIVATEKEY: "$DOCS_PREVIEW_DEPLOY_KEY"
+    DOCS_DEPLOY_PRIVATEKEY: "$DOCS_PREVIEW_PRIVATEKEY"
     DOCS_DEPLOY_SERVER: "$DOCS_PREVIEW_SERVER"
     DOCS_DEPLOY_SERVER_USER: "$DOCS_PREVIEW_SERVER_USER"
     DOCS_DEPLOY_PATH: "$DOCS_PREVIEW_PATH"