Adding `evaluate` to the list of libraries required in generated notebooks (#20850)

Adding `evaluate` to the list of libraries to be installed for every generated notebook in transformers

docs/source/_config.py

 # docstyle-ignore
 INSTALL_CONTENT = """
 # Transformers installation
-! pip install transformers datasets
+! pip install transformers datasets evaluate
 # To install from source instead of the last release, comment the command above and uncomment the following one.
 # ! pip install git+https://github.com/huggingface/transformers.git
 """