docs(user-guide): expand skills marketplace usage and add sharing status screenshots

[alezander86]

Summary

Fills the documentation gap in the Skills Marketplace section and makes use of three previously unreferenced screenshots.

Changes

• Expanded the thin "Add Marketplace Skill to Assistant" section in marketplace-skills.md with proper numbered steps
• Added new "Use a Marketplace Skill in Chat" subsection with step-by-step instructions and screenshot
• Added publish success notification screenshot after the publish confirmation step
• Added skill sharing status badge screenshot to the Sharing Settings table in manage-skills.md

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets (<text> must be `<text>`)
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation

Additional Notes

Relates to EPMCDME-10315.

[github-actions]

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[m-golovchin]

lgtm

[github-actions]

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