Enrich README with background, motivation, findings, and preprint badge

[stedrew]

Closes #10

Summary

• Adds a Research Square preprint badge alongside the CI badge, linking directly to the paper
• Expands the intro paragraph to better convey EDSim's value as a high-fidelity testbed
• Adds Background & Motivation section explaining the clinical problem (ED crowding), the limits of traditional simulation, and how LLM-driven agents fill the gap
• Adds Key Findings & Validation section with three evidence-backed bullets from the preprint (wait-time alignment with real CTAS data, authentic agent behavior, rapid what-if experimentation)
• Updates the Table of Contents to include the two new sections

Test plan

• Verify all internal anchor links in the Table of Contents resolve correctly on GitHub
• Confirm the preprint badge renders and links to the correct Research Square URL
• Read through new sections for clarity and accuracy against the preprint

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