[Documentation] Add documentation for batch 4 components

[djalmaaraujo]

Related Issue

Continuation of documentation migration from the web repository (#336, #337, #338).

Description

This PR adds documentation files for batch 4 components (8 total):

• Popover - Displays rich content in a portal, triggered by a button
• Progress - Displays an indicator showing the completion progress of a task
• RadioButton - A control that allows users to make a single selection from a list
• Select - Displays a list of options for the user to pick from
• Separator - Visually or semantically separates content
• Sheet - Displays content that complements the main content of the screen
• ShortcutKey - A component for displaying keyboard shortcuts
• Sidebar - A composable, themeable and customizable sidebar component

These documentation files are copied from web/app/views/docs/ and placed in lib/ruby_ui/{component}/{component}_docs.rb. They will be distributed to Rails apps via the bin/rails g ruby_ui:install:docs generator.

Test plan

1. Point gem to local source in Gemfile:

   gem "ruby_ui", path: "~/dev/linkana/ruby_ui"

2. Remove existing documentation files for affected components (if any):

   rm app/views/docs/popover.rb
   rm app/views/docs/progress.rb
   rm app/views/docs/radio_button.rb
   rm app/views/docs/select.rb
   rm app/views/docs/separator.rb
   rm app/views/docs/sheet.rb
   rm app/views/docs/shortcut_key.rb
   rm app/views/docs/sidebar.rb

3. Run the docs generator:

   bin/rails g ruby_ui:install:docs

4. Verify all 8 files are created in app/views/docs/

5. Run test suite:

   bundle exec rake