📝 Documentation: Add a tutorial page for `easy_validator`

[sara-czasak]

easy_validator doesn't have a tutorial page yet. This module wraps common input
validation checks — email, username, zip code, URL, and password strength — into
beginner-friendly functions so contributors don't have to hand-write regex.

Please add a short tutorial page demonstrating 2-3 functions used together in a realistic
mini-example — for instance, validating a signup form's email with is_valid_email() and
checking is_password_secure() before "saving" a user.

Match the tone and structure of the existing tutorials in docs/tutorial/ (e.g.
easy_colors.md) — a short intro, a small real-world example, a "What happened?"
breakdown, and a "Why use these helpers?" closer.

Please read CONTRIBUTING.md before you start, and feel free to ask questions here!

Links:

• Module to document: easy_validator.py
• Where the tutorial goes: docs/tutorial/
• Contributing guide: CONTRIBUTING.md