Add ECIES article

[xrissoula]

Description

This PR adds documentation for ECIES (Elliptic Curve Integrated Encryption Scheme) in the Polykey reference docs. This document explains:

• The fundamentals of ECIES and how it works in Polykey
• Why ECIES is used for secure key exchange
• How it integrates with Polykey’s cryptographic model

The document ensures that developers and users understand how ECIES facilitates secure communication and data protection within Polykey.

Issues Fixed

• Relates Create Article in Polykey-Docs Explaining the Polykey Elliptic Curve Article #119

Tasks

• 1. Added ecies.md to docs/reference/
• 2. Ensured formatting aligns with other cryptographic documentation
• 3. Verified technical details for 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