Initial support for Gemini, enables usage of Google LLMs and embedding models (see settings-gemini.yaml)

Install via
poetry install --extras "llms-gemini embeddings-gemini"

Notes:
* had to bump llama-index-core to later version that supports Gemini
* poetry --no-update did not work: Gemini/llama_index seem to require more (transient) updates to make it work...

# Conflicts:
#	poetry.lock
#	pyproject.toml

# Conflicts:
#	fern/docs/pages/manual/llms.mdx