Add Agent example to quickstart page

[julian-risch]

With Agents directly available in Haystack 2.12, we should add an Agent example to the quick start page on the Haystack website by updating the following file: https://github.com/deepset-ai/haystack-home/blob/main/content/overview/quick-start.md

We should align the publication of the updated page with the release of Haystack 2.12 around April 1st and I see it as a high priority issue

In addition I suggest to:

• Split the "Basic RAG pipeline with indexing" under "Build Your First RAG Pipeline" into two because the lines of code look more than necessary.
• We can also put the URL into an extra line of code url = "https://archive.org/stream/leonardodavinci00brocrich/leonardodavinci00brocrich_djvu.txt"
• Replace the text converter with a MultiFileConverter maybe? Means that once people try with a non-txt file they get an error about installing another dependency but that might be okay? We can discuss. We would need to update the pipeline graphs too then.
• The import from haystack.components.builders import PromptBuilder is wrong and it needs to be replaced with from haystack.components.builders import ChatPromptBuilder