docs: Fix Pipeline API design link (#2153)
Hey Turbo folks 👋 ,
Just a tiny fix where a docs link is 404-ing.
| [Prev. Link](https://microsoft.github.io/lage/docs/Guide/pipeline#defining-a-pipeline) | [New Link](https://microsoft.github.io/lage/docs/Tutorial/pipeline#defining-a-pipeline) |
|--------------------------------------------|-------------------------------------------|
|  |  |
Cheers! 🖤
