feat: add Jekyll GitHub Pages setup

Add _config.yml with Cayman theme and index.md that includes
README.md content, enabling GitHub Pages without duplication.

_config.yml

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+title: Penny-Pincher Provider
+description: A curated list of affordable (or almost free) LLM providers
+theme: jekyll-theme-cayman
+show_downloads: false

index.md

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+---
+layout: default
+---
+{% include_relative README.md %}