Add beginner Git and GitHub tutorial examples

[ShawnSiao]

What changed

• add a small browser-based tutorial project
• document the beginner Git workflow used by the video series
• keep the original 2021 repository history unchanged

Why

The example gives first-time Git users a concrete project for commits, diffs, branches, pull requests, conflicts, and safe recovery.

Validation

• git diff --check
• opened tutorial-demo/index.html locally
• confirmed the branch contains only tutorial-demo/ files