[Bug] Enhance the HTTP Auth desc

### Bug Type (问题类型)

gremlin (结果不合预期)

### Before submit

- [x] 我已经确认现有的 [Issues](https://github.com/apache/hugegraph/issues) 与 [FAQ](https://hugegraph.apache.org/docs/guides/faq/) 中没有相同 / 重复问题 (I have confirmed and searched that there are no similar problems in the historical issue and documents)

### Environment (环境信息)

The description regarding the use of Authorization in the official documentation is quite misleading. The term "HTTP plaintext format" is ambiguous.
Furthermore, neither the official website documentation nor the Docker Hub documentation specifies that the default user is "admin". This lack of information is particularly inconvenient for users deploying via Docker.

### Expected & Actual behavior (期望与实际表现)

It would be better to use the example "Authorization: Basic base64(admin:xxxx)" instead, add the default user part in doc.

### Vertex/Edge example (问题点 / 边数据举例)

```javascript

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### Schema [VertexLabel, EdgeLabel, IndexLabel] (元数据结构)

```javascript

```