[docs] Fix website broken anchors

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Description

https://github.com/apache/fluss/actions/runs/24150938424/job/70477440445?pr=2934

```
Warning: Docusaurus found broken anchors!

Please check the pages of your site in the list below, and make sure you don't reference any anchor that does not exist.
Note: it's possible to ignore broken anchors with the 'onBrokenAnchors' Docusaurus configuration, and let the build pass.

Exhaustive list of all broken anchors found:
- Broken anchor on source page path = /docs/0.5/engine-flink/getting-started/:
-> linking to /downloads/#fluss-connector
- Broken anchor on source page path = /docs/0.6/engine-flink/getting-started/:
-> linking to /downloads/#fluss-connector
- Broken anchor on source page path = /docs/next/engine-flink/getting-started/:
-> linking to /docs/next/engine-flink/ddl/#procedures
- Broken anchor on source page path = /docs/engine-flink/getting-started/:
-> linking to /docs/engine-flink/ddl/#procedures
```

### Willingness to contribute

- [ ] I'm willing to submit a PR!

Review the four documentation paths listed in the issue and inspect their links to /downloads/#fluss-connector and /docs/next/engine-flink/ddl/#procedures. Run the Docusaurus check from the linked GitHub Actions job after correcting the references; done means the broken-anchor warnings no longer appear.

Written by the indexing model from the issue text.