Demo Notebook for Using Gemini with VertexAI (#3032)

* add notebook for using Gemini with VertexAI

* add missing image

* remove part with workload identity federation

* Spelling

* Capitalisation and tweak on config note.

* autogen gemini gcp image

* fix formatting

* move gemini vertexai notebook to website/docs/topics/non-openai-models

* Adjust license

Co-authored-by: Chi Wang <[email protected]>

* remove auto-generated cell

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Co-authored-by: Mark Sze <[email protected]>
Co-authored-by: Chi Wang <[email protected]>