Add EfficientDiD v2.7 LinkedIn carousel PDF generator

8-slide dark-themed carousel (4:5 portrait) targeting applied researchers.
Highlights EfficientDiD as the first open-source implementation of
Chen, Sant'Anna & Xie (2025), with slides covering the problem, insight,
PT-All assumption, efficiency payoff, API code, safety net, and CTA.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: igerber