doc: Add to v3-v4 upgrade guide about custom css files

[frayzil]

If an application has custom css files that use tailwind classes like this:

/* app/assets/stylesheets/pagy.css */
@layer components {
  .pagy.info {
    @apply text-gray-600 ml-0 mt-1 text-sm;
  }
}

referenced like this:

/* app/assets/stylesheets/application.tailwind.css */

/* ... */
@import "pagy.css";
/* ... */

Then, the reference has to be changed to:

/* app/assets/stylesheets/application.tailwind.css */

/* ... */
@import "../stylesheets/pagy.css";
/* ... */

for the upstream upgrade tool to run successfully
because the base tailwind css file will be changed
from app/assets/stylesheets/application.tailwind.css
to app/assets/tailwind/application.css

[flavorjones]

Thank you for suggesting this!

[flavorjones]

Thank you again!! ❤️