Remove the validate skill and guidelines doc

[sahaarijit]

Remove the validate skill and the guidelines doc so the sample repo holds only the calendar app.

1. Delete skills/validate/ and docs/, with their .gitattributes export rules.
2. Drop both lines from the README structure tree.
3. Replace the README validation section with a pointer to the assignment guidelines repo.

Why: the skill and the guidelines now live in the guidelines repo for each assignment, so the sample repos stay app specific and each file has one home.