Add package-lock.json fpr the docs build

Without it the build fails with:
```
npm error code EUSAGE
npm error
npm error The `npm ci` command can only install with an existing package-lock.json or
npm error npm-shrinkwrap.json with lockfileVersion >= 1. Run an install with npm@5 or
npm error later to generate a package-lock.json file, then try again.
npm error
npm error Clean install a project
```

https://github.com/apache/avro/actions/runs/18835986364/job/53736989550

Author: martin-g