Add social card / OG image for link previews

[AmitoVrito]

Description

Add Open Graph images so docs links look great when shared on Twitter, LinkedIn, Discord.

What to do

• Create template OG image (1200x630px) with SynapseKit branding
• Configure in docusaurus.config.ts metadata
• Test with Twitter Card Validator and LinkedIn Post Inspector

[AmitoVrito]

Thanks for your interest! I will assign it to you. Before you start, could you give the repo a ⭐ on the main repo (https://github.com/SynapseKit/SynapseKit)? It really helps with visibility for a new project. Looking forward to your PR!

[AmitoVrito]

Done — og:image, twitter:image, og:type meta tags added pointing to banner.svg in PR #44.