docs: Add missing Servant pattern to README behavioral patterns table

[xingarr]

Description

This PR adds the missing Servant design pattern entry to the README's Behavioral Patterns table.

Problem

The Servant pattern was fully implemented in patterns/behavioral/servant.py with:

• ✅ Complete implementation with proper docstrings
• ✅ Working examples and doctests
• ✅ Unit tests in tests/behavioral/test_servant.py

However, it was not documented in the README.md's Behavioral Patterns table, making it invisible to users browsing the repository.

Solution

Added the Servant pattern entry to the Behavioral Patterns table in alphabetical order:

| [servant](patterns/behavioral/servant.py) | provide common functionality to a group of classes without using inheritance |