docs: generate dynamic OG images at build time

- Add satori + @resvg/resvg-js for build-time image generation
- Each page gets a branded 1200x630 PNG with title, description, and category
- Blue gradient background with white Vuetify icon watermark
- Hook into SSG plugin to generate images and inject per-page og:image meta
- Update metadata.ts to support per-page og:image in OG and Twitter tags