docs: reflect wider validate-cards trigger in CONTRIBUTING.md

[aditya-pola]

The workflow now runs on every pull request to main, not only ones that touch cards/. Update the text in CONTRIBUTING.md so the description matches the actual trigger.

What does this PR do?

Checklist

For new cards

• Card file is in cards/ with filename matching the id field
• id is lowercase kebab-case
• category exists in cards/_categories.json
• oneliner is 80-160 characters (ideal), 40-200 allowed (automated check)
• explanation is 300-480 characters (ideal), 150-500 allowed (automated check)
• explanation starts with a plain-language "what is this" sentence (automated check)
• No LaTeX ($) in explanation — formulas go in fundamentals only (automated check)
• related contains only existing card IDs (automated check)
• resources contains at least 1 external link with url and label
• verified_date is set to today in YYYY-MM-DD format
• I have read the Contributing Guide

Automated checks run on every PR. The PR will fail if any card has invalid JSON, unknown categories, broken related links, wrong field lengths, or LaTeX in the explanation.

For card edits

• Changes follow the quality rubric
• No broken related links introduced

For new categories

• Added to cards/_categories.json with label and order
• At least one card uses the new category