Document the configuration mechanism

[jlamypoirier]

🎯 Goal (What & Why)

Add developer documentation for the config mechanism.

🚀 Execution Plan

Basic topics:

• Create a basic config class: fields, validation,
• Create a nested config config class: array, dict, config fields; cross-config validation
• Create a derived config class: field update
• Manipulate config objects: instantiate, copy, serialize
• Create a configurable class

Advanced topics:

• Explicit and implicit fields
• Update and override configs

📌 Acceptance Criteria (Must-Haves for Completion)

• The feature must be functional and tested.
• The implementation must be documented in practical terms.
• The PR must include a performance/impact summary.
• No refactors unless directly necessary for feature completion.

🛠️ Project Management

• Assign the project to the Fast-LLM project.
• Set the Estimate field (in days) in the GitHub project.
• Use the Size field to categorize the PR size (Small/Medium/Large).
• Assign an owner when opening the issue.