Add notebook with example code for LLMMetadataExtractor

[julian-risch]

We added LLMMetadataExtractor in #125 and recently updated it in #147

What's still missing is a notebook showing how to use it, which we need to link in the experiment catalogue in the repo's readme file. Currently it says coming soon.

Maybe something related to https://colab.research.google.com/github/deepset-ai/haystack-cookbook/blob/main/notebooks/metadata_enrichment.ipynb ?

[sjrl]

Here is an example yaml that could help you all get started.

It extracts the company name from text versions of Slide decks.

I had to change the file format to .txt even though it is yaml since GH doesn't support yaml files.

haystack-metadata-extraction-example.txt