feat: Add new slots for customizing course outline section and sequence completion icons (#1933)

* feat: Add new slots for customizing course outline section and sequence completion icons

Introduces `CourseOutlineSidebarSectionCompletionIconSlot` and `CourseOutlineSidebarSequenceCompletionIconSlot` to enable customization, modification, or replacement of completion icons in the course outline. Updated components to integrate with these new slots.

* fixup! feat: Add new slots for customizing course outline section and sequence completion icons

* feat: Add `active` prop to completion icon slots

Updated `CourseOutlineSidebarSectionCompletionIconSlot` and `CourseOutlineSidebarSequenceCompletionIconSlot` to include an `active` prop, enabling styling based on the active state of sections or sequences.

* refactor: Consolidate section and sequence completion icon slots

Combined `CourseOutlineSidebarSectionCompletionIconSlot` and `CourseOutlineSidebarSequenceCompletionIconSlot` into a single `CourseOutlineSidebarCompletionIconSlot` to simplify implementation. Updated components to use the `variant` prop for distinguishing between sections and sequences.docs: Add README for `CourseOutlineSidebarCompletionIconSlot`

Introduced documentation for the consolidated `CourseOutlineSidebarCompletionIconSlot`, including usage examples for customizing section and sequence completion icons.

* fix: apply review feedback