Feature: Add model.description

[ondrejch]

Description

This pull request introduces a description attribute to the Model class, allowing users to add a descriptive text to their models. This description is saved in the settings.xml output.

Key changes:

• Added description property to openmc.model.Model.
• Updated openmc.settings.Settings to include and handle the description in XML exports.
• Added documentation for the new feature.
• Included unit tests to verify the new functionality.

Fixes # 3586

Checklist

• I have performed a self-review of my own code
• [N/A] I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)