[Documentation:TAGrading] Multi-Grader Annotations

[Imfishm]

Why is this Change Important & Necessary?

Documentation change for Submitty/Submitty#13460.

Previously, when more than one grader annotated the same image, students only saw one grader's annotations. That PR lets students view and download annotations from every grader, so the Image Annotation page needs to describe this.

What is the New Behavior?

Adds a short Viewing Annotations from Multiple Graders section to the Image Annotation page, explaining that:

• View Popup has a button for each grader plus an All Graders button that combines them (with a warning that annotations may overlap).
• Download asks which version to save: a single grader's annotations or All Graders Combined.
• With only one grader, View Popup shows the annotated image and Download saves it directly.

Two new screenshots were added to images/ta_grading/:

• rubric_grading_image_annotation_all_graders.png
• rubric_grading_image_annotation_download.png

What steps should a reviewer take to reproduce or test the bug or new feature?

1. Check out this branch and run docker compose up.
2. Visit http://localhost:4000/grader/rubric_grading/image_annotation and confirm the new section and both images render.

Automated Testing & Documentation

jekyll build and htmlproofer (the same link check run in CI) pass locally.

Other information

Related: Submitty/Submitty#13156, Submitty/Submitty#13460