Fix: APISIX Dashboard "etcd not connected" issue in Docker Compose setup

[Sahil2k24]

Description:

This pull request enhances the APISIX Docker setup by introducing the APISIX Dashboard for easier management and visualization. The following key changes were made:

- Added example/dashboard_conf/conf.yaml
A new configuration file for the APISIX Dashboard has been created. This file includes the necessary settings to connect to the etcd instance and manage APISIX through a web interface.

### - Updated docker-compose.yml
A new service named apisix-dashboard has been added to the Docker Compose configuration. This service runs the APISIX Dashboard and is configured to connect to the existing etcd instance on etcd:2379.

### - etcd Connection Fix
The configuration ensures the dashboard connects to etcd correctly using the internal Docker network.

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