docs: add NestJS integration example

[Muzzu8421]

This PR adds a NestJS integration example for Casbin using a custom guard.

It demonstrates how to use Casbin for authorization in a NestJS application, including installation, guard implementation, and policy configuration.

The example shows how to protect routes using a NestJS guard and enforce access control with Casbin.

This will help developers easily integrate Casbin with modern Node.js frameworks like NestJS.

[hsluoyz]

Thanks for the contribution! A couple of things though: node-casbin doesn't host usage guides — all docs live in the casbin-website repo (casbin.apache.org/docs), so a docs/ file here wouldn't be linked or discoverable. We also already have an official NestJS integration, nest-authz (https://github.com/apache/casbin-nest-authz) with a full example repo, so a guide should build on that rather than a hand-rolled guard. The example itself also has a few issues worth fixing first: the subject is hardcoded to 'alice' instead of read from request.user, request.url includes the query string so /data?page=1 won't match the /data policy (RESTful paths need keyMatch2), the enforcer is created in an async onModuleInit without implements OnModuleInit (racy — better to provide the Enforcer as a useFactory provider and inject it), and every code block is tagged ```bash even for TS/conf/csv. I'd suggest closing this and opening a PR against casbin-website instead. Thanks again!