docs: update SC-500 study guide link

[lovewave02]

Summary

• updates the stale Microsoft Learn SC-500 exam URL to the current study guide URL
• fixes the README badge target and Official study guide row
• keeps the related docs/resources and issue-template contact link aligned

Fixes #1.

Verification

• Confirmed the old https://learn.microsoft.com/credentials/certifications/exams/sc-500/ URL returns 404.
• Confirmed https://learn.microsoft.com/en-us/credentials/certifications/resources/study-guides/sc-500 returns 200.
• grep -RIn "https://learn.microsoft.com/credentials/certifications/exams/sc-500" . --exclude-dir=.git || true
• git diff --check

AI assistance disclosure

This PR was prepared with AI assistance. I reviewed the changed links and verification output before submission.