feat: improve the quick start section of the readme (halo-dev#2899)

#### What type of PR is this?

/kind documentation

#### What this PR does / why we need it:

更新 Readme 中快速开始的命令，提供初始管理员账号和外部访问链接的环境变量设置。

```release-note
None
```

README.md

@@ -24,10 +24,18 @@
 ## 快速开始
 
 ```bash
-docker run -it -d --name halo-next -p 8090:8090 -v ~/.halo2:/root/.halo2 --restart=unless-stopped halohub/halo:2.0.1
+docker run \
+  -it -d \
+  --name halo \
+  -p 8090:8090 \
+  -v ~/.halo2:/root/.halo2 \
+  -e HALO_EXTERNAL_URL=http://localhost:8090/ \
+  -e HALO_SECURITY_INITIALIZER_SUPERADMINUSERNAME=admin \
+  -e HALO_SECURITY_INITIALIZER_SUPERADMINPASSWORD=P@88w0rd \
+  halohub/halo:2.0
 ```
 
-详细部署文档请查阅：<https://docs.halo.run/getting-started/install/docker-compose>
+以上仅作为体验使用，详细部署文档请查阅：<https://docs.halo.run/getting-started/install/docker-compose>
 
 ## 在线体验