docs: logo alt attr documentation (#1415)

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

docs/guide/theme-nav.md

@@ -35,6 +35,8 @@ export default {
 }
 ```
 
+You can also pass an object as logo if you want to add `alt` attribute or customize it based on dark/light mode. Refer [`themeConfig.logo`](../config/theme-configs#logo) for details.
+
 ## Navigation Links
 
 You may define `themeConfig.nav` option to add links to your nav.