feat: make studio course outline help sidebar collapsible with header

[choeying-10]

Summary

• Converts the four help sections in the course outline sidebar ("Creating your course organization", "Reorganizing your course", "Setting release dates and grading policies", "Changing the content learners see") into collapsible items, collapsed by default, to reduce visual clutter.
• Adds a "Need help with your course?" header above the collapsibles.
• Aligns the header and collapsibles with the content column (StatusBar / section card top border) and applies consistent styling: blue header, black collapsible titles with right-aligned chevrons, no underline.

Changes

• src/course-outline/outline-sidebar/OutlineSidebar.jsx: render header + wrap sections in Paragon Collapsible (styling="basic", unmountOnExit={false}); remove old <hr> dividers and mt-4.
• src/course-outline/outline-sidebar/messages.js: add sidebar_header i18n message.
• src/generic/help-sidebar/HelpSidebar.scss: scoped .outline-sidebar styles for header, section spacing/alignment, right-aligned chevrons, underline removal, and colors.