add(self extend): add self extend documentation

Author: hahuyhoang411

docs/docs/features/grammar.md

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+---
+title: GBNF Grammar
+description: What Nitro supports
+keywords: [Nitro, Jan, fast inference, inference server, local AI, large language model, OpenAI compatible, open source, llama]
+---

docs/docs/features/self-extend.md

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+---
+title: Self extend
+description: Self-Extend LLM Context Window Without Tuning
+keywords: [long context, longlm, Nitro, Jan, fast inference, inference server, local AI, large language model, OpenAI compatible, open source, llama]
+---
+
+## Enhancing LLMs with Self-Extend
+Self-Extend offers an innovative approach to increase the context window of Large Language Models (LLMs) without the usual need for re-tuning. This method adapts the attention mechanism during the inference phase and eliminates the necessity for additional training or fine-tuning.
+
+For in-depth technical insights, refer to their research [paper](https://arxiv.org/pdf/2401.01325.pdf).
+
+## Activating Self-Extend for LLMs
+
+To activate the Self-Extend feature while loading your model, use the following command:
+
+```bash title="Enable Self-Extend" {6,7}
+curl http://localhost:3928/inferences/llamacpp/loadmodel \
+  -H 'Content-Type: application/json' \
+  -d '{
+    "llama_model_path": "/path/to/your_model.gguf",
+    "ctx_len": 8192,
+    "grp_attn_n": 4,
+    "grp_attn_w": 2048,
+  }'
+```
+
+**Note:** 
+- For optimal performance, `grp_attn_w` should be as large as possible, but smaller than the training context length.
+- Setting  `grp_attn_n` between 2 to 4 is recommended for peak efficiency. Higher values may result in increased incoherence in output.