docs: add NestJS integration example

docs/nestjs.md

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+# Using Casbin with NestJS
+
+This guide demonstrates how to integrate Casbin with a NestJS application using a custom guard for authorization.
+
+## Installation
+
+Install Casbin in your NestJS project:
+
+```bash
+npm install casbin
+```
+
+# Create a Casbin Guard
+
+Create a guard to handle authorization using Casbin.
+```bash
+import {
+  Injectable,
+  CanActivate,
+  ExecutionContext,
+} from '@nestjs/common';
+import { newEnforcer } from 'casbin';
+
+@Injectable()
+export class CasbinGuard implements CanActivate {
+  private enforcer: any;
+
+  async onModuleInit() {
+    this.enforcer = await newEnforcer(
+      'basic_model.conf',
+      'basic_policy.csv'
+    );
+  }
+
+  async canActivate(context: ExecutionContext): Promise<boolean> {
+    const request = context.switchToHttp().getRequest();
+
+    const user = 'alice'; // example user
+    const resource = request.url;
+    const action = request.method;
+
+    return await this.enforcer.enforce(user, resource, action);
+  }
+}
+```
+
+# Use Guard in Controller
+
+Apply the guard to protect routes.
+```bash
+import { Controller, Get, UseGuards } from '@nestjs/common';
+import { CasbinGuard } from './casbin.guard';
+
+@Controller()
+export class AppController {
+
+  @UseGuards(CasbinGuard)
+  @Get('data')
+  getData() {
+    return 'Protected Data';
+  }
+}
+```
+
+# Model Configuration
+
+Create a file named basic_model.conf:
+
+```bash
+[request_definition]
+r = sub, obj, act
+
+[policy_definition]
+p = sub, obj, act
+
+[policy_effect]
+e = some(where (p.eft == allow))
+
+[matchers]
+m = r.sub == p.sub && r.obj == p.obj && r.act == p.act
+```
+
+# Policy Example
+
+Create a file named basic_policy.csv:
+
+```bash
+p, alice, /data, GET
+```
+
+# How It Works
+- Casbin loads the model and policy.
+- NestJS guard intercepts incoming requests.
+- The enforce function checks permissions.
+- If access is allowed, the request proceeds.
+- If denied, the request is blocked.
+
+# Example Result
+
+When accessing:
+```GET /data```
+- User alice → Allowed
+- Other users → Denied