Document how to upgrade subtheme

Problem/Motivation

While this project is in active development, users will want to perform updates to their sub-theme. The process of comparing the base theme sub-theme kit against a project sub-theme can be tricky. Adding an automated solution may not be doable, as there are any number of customizations a project may make after sub-theme is created.

Proposed resolution

I presented what I think is a simple solution in the Radix issue queue. I suggest we include this as part of documentation.

Remaining tasks

Add the following steps to documentation:

1. Update the contrib theme.

   composer update drupal/bootstrap_storybook
   

2. Make sure bootstrap_storybook is enabled so we can use the sub-theme create command.

   drush en bootstrap_storybook
   

3. Set default theme to something else so we can temporarily uninstall it and move it

   drush config-set system.theme default bartik
   drush theme:uninstall my_bootstrap_storybook
   mv web/themes/custom/my_bootstrap_storybook ../my_bootstrap_storybook.bak
   drush --include="web/themes/contrib/bootstrap_storybook" bootstrap_storybook:create "My Bootstrap Storybook Theme"
   

4. Have a look at the differences, and make adjustments as necessary.

   git diff -- . ':(exclude)web/themes/custom/my_bootstrap_storybook/package-lock.json'
   

5. Make sure things run smoothly.

   npm install
   npm run dev
   npm run watch
   drush theme:enable my_bootstrap_storybook
   drush config-set system.theme default my_bootstrap_storybook
   

6. If all is well, commit your changes.