refactor: download all images in-repo instead of user-attachments

[FidelusAleksander]

Summary

This pull request updates the documentation and workflow files to improve the clarity and reliability of the Codespaces setup instructions. The main focus is on replacing all image links in the markdown steps with repository-hosted images for better stability, and updating the workflow to use a specific branch reference.

Documentation improvements:

• All image links in the .github/steps/*.md files have been updated to use raw GitHub URLs pointing directly to images stored in the repository, instead of user-uploaded assets. This ensures that the images remain accessible and consistent regardless of external changes. [1] [2] [3] [4] [5] [6] [7] [8]

Workflow update:

• The start-exercise job in .github/workflows/0-start-exercise.yml now references the start-exercise-updates branch of the skills/exercise-toolkit workflow, instead of a fixed version tag, allowing for use of the latest updates from that branch.

Changes

Closes:

Task list

• For workflow changes, I have verified the Actions workflows function as expected.
• For content changes, I have reviewed the style guide.