[doc] fix the incorrect logo in dark mode (#20289)

Signed-off-by: reidliu41 <reid201711@gmail.com>

docs/README.md

 # Welcome to vLLM
 
 <figure markdown="span">
-  ![](./assets/logos/vllm-logo-text-light.png){ align="center" alt="vLLM" class="no-scaled-link" width="60%" }
+  ![](./assets/logos/vllm-logo-text-light.png){ align="center" alt="vLLM Light" class="logo-light" width="60%" }
+  ![](./assets/logos/vllm-logo-text-dark.png){ align="center" alt="vLLM Dark" class="logo-dark" width="60%" }
 </figure>
 
 <p style="text-align:center">

docs/mkdocs/stylesheets/extra.css

@@ -134,3 +134,12 @@ body[data-md-color-scheme="slate"] .md-nav__item--section > label.md-nav__link .
   opacity: 0.9;
   transform: translateY(2px);
 }
+
+/* For logo css */
+[data-md-color-scheme="default"] .logo-dark {
+  display: none;
+}
+
+[data-md-color-scheme="slate"] .logo-light {
+  display: none;
+}