Use npm ci instead of npm install for themes

xref: jupyter-book/mystmd#2487

Author: stefanv

themes/article/template.yml

@@ -65,7 +65,7 @@ options:
     id: style
     description: Local path to a CSS file
 build:
-  install: npm install
+  install: npm ci
   start: npm run start
 files:
   - server.js

themes/book/template.yml

@@ -78,7 +78,7 @@ parts:
     description: The primary sidebar footer, to replace Made with MyST
     required: false
 build:
-  install: npm install
+  install: npm ci
   start: npm run start
 files:
   - server.js