assessment update (#435)

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## Summary by CodeRabbit

* **Documentation**
* Added Regenerate path in AI assessment generation (step 4) with
guidance and example prompt.
* Added Starter Pack mention and install steps for assessment examples.
* Expanded Random Assessments: layout requirements, duplication
prevention, sync/publish workflows, filter categories/table, and updated
images.
* Reorganized SPLICE, Standard Code Test, and Student Submission guides
(structure, tabs/examples, submit/mark-as-complete flows,
wording/formatting).
  * Clarified ungraded-assessments guidance and minor file cleanup.
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Author: LolaValente