Update Userguide quick start docs with frontmatter and editorial improvements

[satyaog]

Summary

Adds YAML frontmatter (title + description) to the three Quick Start guide pages
and applies editorial polish throughout — clearer wording, minor restructuring,
and formatting fixes.

Changes

• Added title and description frontmatter to Userguide_quick_start.md,
  Userguide_quick_start_my_first_job.md, and
  Userguide_quick_start_train_first_model.md.
• Reformatted Key Concepts sections: switched from bullet lists to definition
  lists in the Quick Start and Train guides.
• Converted a plain prerequisite note to a !!! success admonition with clearer
  checklist in the Quick Start guide.
• Replaced a vague failure note with an !!! failure admonition for the SSH
  connection check.
• Changed !!! note to !!! success for the VSCode prerequisite callout in the
  First Job guide.
• Swapped bare - spacers for   in the First Job and Train guides.
• Minor wording improvements throughout (passive voice reduction, line length,
  removed second-person "you will").

[satyaog]

The skills from #352 have been used to update these guides