Update CSS Variables and Remove Unused Components

[cirdes]

Why:

Variables on shadcn are different from the current rbui variables. Some are missing and others has different values.

You can check the default theme variables here: https://ui.shadcn.com/themes

This PR includes the following changes:

• CSS Updates: Modified the CSS variables in application.tailwind.css to update color values and add new variables for card and popover based on Shadcn
• Remove General Sans as default font-sans
• Component Cleanup: Removed unused components installation_instructions_coming_soon.rb and premium_badge.rb.
• Documentation Updates: Updated rails_bundler_view.rb and rails_importmaps_view.rb in the installation documentation.
• Configuration Update: Modified tailwind.config.js to reflect the new changes.