docs(research): add Submission Cover Letter template (#415)

Complete template for conference submissions:
- 4 conference-specific variations (NeurIPS, ICLR, MLSys, AAAI)
- 4 category templates (method, system, theory, empirical)
- Writing guidelines and checklist
- Email vs PDF formats
- Example cover letters included

~1,200 LOC

Author: Antigravity Agent