Jupyter notebook new cell type default

Go to the site-packages directory of your python environment and modify site-packages/notebook/static/notebook/js/main.min.js

Find something like

Notebook.options_default = {
    // can be any cell type, or the special values of
    // 'above', 'below', or 'selected' to get the value from another cell.
    default_cell_type: 'code'
};

change default_cell_type: 'code' into default_cell_type: 'markdown'

In JupyterLab, you can go to Settings -> Advanced Settings manager -> Notebook and then add the following in "User Preferences":

{
    "defaultCell": "markdown"
}