add social sharing metadata (opengraph/twitter) and configurable SEO Layer

[ruhilmansi]

the site currently includes only a basic meta description and lacks structured social metadata

also SEO values are hardcoded in index.html, making them difficult to update for future conference editions

solution: introduce a small and configurable SEO layer and add full social metadata support without changing the existing tech stack

scope of work:

1. add opengraph metadata
2. add twitter card support
3. add canonical URL
4. move SEO content into config file