[ci] cache deps for web export

.circleci/config_tmpl.yml

@@ -134,9 +134,16 @@ jobs:
       - run:
           name: Initialize bot account
           command: .circleci/bot_init
+      - restore_cache:
+          keys:
+            - webe-dps-{{ .Branch }}-{{ checksum ".circleci/web/.spacemacs" }}
       - run:
           name: Install dependencies
           command: .circleci/web/install_deps
+      - save_cache:
+          key: webe-dps-{{ .Branch }}-{{ checksum ".circleci/web/.spacemacs" }}
+          paths:
+            - ~/.emacs.d/elpa
       - run:
           name: Export documentation files
           command: .circleci/web/htmlize