Add KEM article

[xrissoula]

Description

This PR adds documentation for Key Encapsulation Mechanism (KEM) in Polykey, covering:

• How KEM enables secure key exchange in hybrid cryptography
• The difference between KEM and traditional key exchange methods
• Why Polykey uses Elliptic Curve KEM (EC-KEM) for better security

This document ensures that developers understand how Polykey protects symmetric keys in a hybrid encryption system.

Issues Fixed

• Relates Create Article in Polykey-Docs Explaining the Polykey Elliptic Curve Article #119
• Provides a reference for secure key exchange mechanisms
• Documents Polykey’s approach to key encapsulation

Tasks

• 1. Added kem.md to docs/reference/
• 2. Ensured formatting consistency
• 3. Verified cryptographic accuracy

Final checklist

• Domain specific tests
• Full tests
• Updated inline-comment documentation
• Lint fixed
• Squash and rebased
• Sanity check the final build

[xrissoula]

This PR is closed in favor of #135