* 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
* Add nbsphinx extension for docs
* Add specific ipynb files to readthedocs folder and add links to Nano Tutorials index page
* Disable execution of ipynb notebook
* Remove outputs of two ipynb files
* Rename titles and add navigation texts to seq_and_func.ipynb under readthedocs folder
* Rename titles and add navigation texts to custom.ipynb file under readthedocs folder