add colab button to _conf.yml

[mikofski]

Similar to other tutorials, EG: see PV-Tutorials/pyData-2021-Solar-PV-Modeling#4 add the colab option to the jupyter ebook and add a code cell to install requirements like this:

# if running on google colab, uncomment the next line and execute this cell to install the dependencies and prevent "ModuleNotFoundError" in later cells:
# !pip install -r https://raw.githubusercontent.com/PVSC-Python-Tutorials/<repo-name>/main/requirements.txt