docs: add type description of hero image (#1691)

Co-authored-by: Divyansh Singh <40380293+brc-dd@users.noreply.github.com>

docs/guide/theme-home-page.md

@@ -48,10 +48,18 @@ interface Hero {
   // Tagline displayed below `text`.
   tagline?: string
 
+  // The image is displayed next to the text and tagline area.
+  image?: ThemeableImage
+
   // Action buttons to display in home hero section.
   actions?: HeroAction[]
 }
 
+type ThemeableImage =
+  | string
+  | { src: string; alt?: string }
+  | { light: string; dark: string; alt?: string }
+
 interface HeroAction {
   // Color theme of the button. Defaults to `brand`.
   theme?: 'brand' | 'alt'