Update recipes in Haystack docs

[julian-risch]

A community member pointed me to outdated code in one of our recipes: https://github.com/deepset-ai/haystack/discussions/8299#discussion-7103259 That particular recipe is fixed now but we should check and update all others too. For example, the "Getting Started Try out a simple pre-build Pipeline!" seems outdated too as there is no build_rag_pipeline in Haystack anymore. We need to update the code and adjust the highlighted lines of code if necessary.

https://docs.haystack.deepset.ai/v2.5-unstable/recipes

### Tasks
- [x] 2.0 RAG Pipeline
- [ ] Custom Components in a Pipeline
- [ ] Getting Started
- [ ] TopPSampler in a Pipeline