[Feature] Add list_course_rubrics() to vvcanvas

[tin900]

Summary:
Implement a function to retrieve all rubrics attached to a course.

What to implement:

• Function: list_course_rubrics(course_id, ...)
• Endpoint: GET /api/v1/courses/:course_id/rubrics
• Should handle optional params:
  • include (string or vector; e.g., "associations", "rubric_assessments")
  • Pagination (as per vvcanvas conventions)
• Return: List/dataframe of rubric objects with all available metadata

Design notes:

• Follow naming and parameter conventions used in other list_* vvcanvas functions.
• Add @examples and detailed param docs in Roxygen style.
• Add endpoint reference in docs.
• Add to pkgdown _pkgdown.yml under reference index.

To update:

• Add entry in TODO.md
• Add to _pkgdown.yml under "Rubrics" or new section if appropriate.

[linear]

EDU-160 [Feature] Add list_course_rubrics() to vvcanvas