fix: add docs/package-lock.json for VitePress CI build

The root .gitignore had a blanket `package-lock.json` rule that prevented
the docs lock file from being committed. Change it to `/package-lock.json`
(root-only) and commit the generated lock file so `npm ci` and
setup-node cache work in the docs workflow.

.gitignore

@@ -12,7 +12,7 @@ node_modules/
 /app/app
 /app/google
 /app/gradle.properties
-package-lock.json
+/package-lock.json
 .idea/
 # Kotlin 2.0
 .kotlin/