docs: add different logos for light and dark modes (#1155)

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

docs/config/theme-configs.md

@@ -21,9 +21,9 @@ Here it describes the settings for the VitePress default theme. If you're using
 
 ## logo
 
-- Type: `string`
+- Type: `ThemeableImage`
 
-Logo file to display in nav bar, right before the site title.
+Logo file to display in nav bar, right before the site title. Accepts a path string, or an object to set a different logo for light/dark mode.
 
 ```ts
 export default {
@@ -33,6 +33,11 @@ export default {
 }
 ```
 
+```ts
+type Image = string | { src: string; alt?: string }
+type ThemeableImage = Image | { light: Image; dark: Image }
+```
+
 ## siteTitle
 
 - Type: `string | false`