* Update open in colab button from HTML-style hyperlinks to markdown image with link for notebooks in readthedocs folder
* Update "Open in Colab" button for notebooks under python folder with markdown image with link
* how to train forecaster
* add a notebook
* add new notebook
* add index
* fix colab link
* change content
Co-authored-by: Xinyi Zhang <xinyi1.zhang@intel.com>
Co-authored-by: theaperdeng <theaperdeng@outlook.com>