docs: add comprehensive SEO subpages research and proposal
Research findings from user analytics, competitor analysis, and SEO trends
to propose strategic subpages for improving HuggingChat's organic visibility.
Key recommendations:
- Use case pages (coding, writing, research, summarization)
- Comparison pages vs ChatGPT, Claude, Gemini, etc.
- Alternative pages targeting competitor keywords
- Feature pages (web-search, multi-model, no-login, privacy)
- Technical SEO foundation (sitemap, structured data, schemas)
