docs: add mistral/labs-leanstral-1-5 (#280)

Author: StanAIML

docs/SUMMARY.md

@@ -127,6 +127,7 @@
     * [M3](api-references/text-models-llm/minimax/m3.md)
   * [Mistral AI](api-references/text-models-llm/Mistral-AI/README.md)
     * [Mistral Nemo](api-references/text-models-llm/Mistral-AI/mistral-nemo.md)
+    * [Leanstral 1.5](api-references/text-models-llm/Mistral-AI/labs-leanstral-1-5.md)
   * [Moonshot](api-references/text-models-llm/moonshot/README.md)
     * [kimi-k2-preview](api-references/text-models-llm/moonshot/kimi-k2-preview.md)
     * [Kimi k2.5](api-references/text-models-llm/moonshot/kimi-k2-5.md)