feat: replace relative links with raw.githubusercontent in start-exercise workflow

[FidelusAleksander]

Changes

This pull request streamlines the workflow for starting an exercise by merging the previously separate "update_readme" job into the "create_exercise" job, ensuring that both issue creation and README updates happen in a single job. It also introduces a step to update image URLs in step files allowing to store images in repo instead of user attachments and still work fine for Skills issue flow

Workflow consolidation and improvements:

• Merged the "update_readme" job into the "create_exercise" job, so that creating the exercise issue and updating the README now occur together, simplifying the workflow and reducing duplication. [1] [2]
• Changed the job name to "Create exercise issue and update README" to reflect the consolidated responsibilities.

Documentation updates:

• Added a new step to find and replace relative image URLs in .github/steps/*.md files with absolute URLs, ensuring images render correctly regardless of context.
• Updated the commit step to include both README.md and all step markdown files, so changes to documentation are committed together.

Variable and output handling:

• Fixed the variable reference for issue_url in the template-vars section to use the correct step output, ensuring the README contains the right link.

Checklist

• I have added or updated appropriate labels to this PR
• I have tested my changes
• I have updated the documentation if needed

[Copilot]

Pull request overview

This PR updates the “Start Exercise” reusable workflow to consolidate README updates into the issue-creation job, and to rewrite relative image links in step markdown so they render correctly outside the repository context (e.g., in Skills issue flows).

Changes:

• Merged the previous update_readme job into create_exercise, so issue creation and README updates occur in a single job.
• Added a find/replace step to convert ../images/ references in .github/steps/*.md to absolute raw.githubusercontent.com URLs.
• Fixed the issue_url variable source to use the create-issue step output.

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[arilivigni]

LGTM