0.70.6

Add custom OG image for the landing page (#172)

* Add custom OG image for the landing page

- New docs/src/app/(home)/opengraph-image.tsx generated via
  @takumi-rs/image-response (already used for /og/docs).
- White 1200x630 canvas: OptimusKG logo + wordmark top-left, tagline
  "Unifying biomedical knowledge in a modern multimodal graph"
  bottom-left, both set in Inter (fetched from fontsource jsDelivr
  at prerender time).

* Shrink OG tagline to 60px so it fits on two lines

* Match OG tagline weight to the landing hero (Inter 400)