[Frontend] Support reasoning content for deepseek r1 (#12473)

Signed-off-by: Ce Gao <cegao@tensorchord.ai>
Co-authored-by: Rafael Vasquez <rafvasq21@gmail.com>
Co-authored-by: Cyrus Leung <cyrus.tl.leung@gmail.com>
Co-authored-by: Michael Goin <mgoin@redhat.com>

docs/source/index.md

@@ -90,6 +90,7 @@ models/extensions/index
 features/quantization/index
 features/lora
 features/tool_calling
+features/reasoning_outputs
 features/structured_outputs
 features/automatic_prefix_caching
 features/disagg_prefill