[Design Doc] Add initial design document for llm-eval-kit

[varishasiddiqui]

What does this PR do?

Adds the initial design document for the llm-eval-kit project, including the project summary, problem statement, technical approach, module ownership table, and known risks.

Which module(s) does it touch?

• docs/design_document.md – Added the initial project design document containing Sections 1, 2, 3, 5, and 6.

How did you test it?

Documentation-only change. No code or tests were added or executed.

Edge cases handled:

Not applicable. This PR only adds project documentation and does not introduce executable code.

Checklist:

• Tests added or updated
• Docstrings on all new public functions
• No unused imports
• README updated if public API changed
• Lead Engineer has reviewed and approved before submitting to director

[varishasiddiqui]

@MahrukhBaig, This is my draft of the design document (Sections 1, 2, 3, 5, and 6). Happy to incorporate feedback or merge useful parts into the final document.