docs: add LLM fixtures for demo dataset (fine-tuning dataset), fix demo notebook (#4286)

* wip

* wip

* wip

* traces/evals

* updated and fixed

* fix names

* fixtures

* transfer fixtures outsiide of git

* expose new demo data as trace dataset fixture

* remove unnecessary notebook

* remove json files and add to .gitignore

* rename directory/files, fix .gitignore

* rename

* fix notebook and fixtures with renamed files

* missed a section

* post-review changes

* demo llm fixtures, fix notebook

* fix comments