[Design Doc] Combined structure with Section 3 draft

[Maazkorejo]

What does this PR do?

Sets up a single combined design_doc.md with all 7 sections scaffolded, since the team agreed to consolidate into one file instead of separate section PRs. Section 3 (Technical Approach) is fully drafted with real test data. Other sections are placeholders with owner tags for whoever picks them up.

Which module(s) does it touch?

docs/design_doc.md — new file. No source code, per the Week 1 no-code rule.

How did you test it?

N/A, documentation only.

Edge cases handled:

N/A for this PR — the document itself discusses edge cases for the four evaluation criteria within Section 3.

Checklist:

• Tests added or updated — N/A
• Docstrings on all new public functions — N/A
• No unused imports — N/A
• README updated if public API changed — N/A
• Lead Engineer has reviewed and approved before submitting to director — pending, this is the shared scaffold for the team to build on