add(grammar): add documentation

Author: hahuyhoang411

docs/docs/features/grammar.md

@@ -2,4 +2,32 @@
 title: GBNF Grammar
 description: What Nitro supports
 keywords: [Nitro, Jan, fast inference, inference server, local AI, large language model, OpenAI compatible, open source, llama]
----
+---
+
+## GBNF Grammar
+
+GBNF (GGML BNF) makes it easy to set rules for how a model talks or writes. Think of it like teaching the model to always speak correctly, whether it's in emoji or proper JSON format.
+
+Bakus-Naur Form (BNF) is a way to describe the rules of computer languages, files, and how they talk to each other. GBNF builds on BNF, adding modern features similar to those found in regular expressions.
+
+In GBNF, we create rules (production rules) to guide how a model forms its responses. These rules use a mix of fixed characters (like letters or emojis) and flexible parts that can change. Each rule follows a format: `nonterminal ::= sequence...`.
+
+To get a clearer picture, check out [this guide](https://github.com/ggerganov/llama.cpp/blob/master/grammars/README.md).
+
+## Use GBNF Grammar in Nitro
+
+To make your Nitro model follow specific speaking or writing rules, use this command:
+
+```bash title="Nitro Inference With Grammar" {9}
+curl http://localhost:3928/v1/chat/completions \
+  -H "Content-Type: application/json" \
+  -d '{
+    "messages": [
+      {
+        "role": "user",
+        "content": "Who won the world series in 2020?"
+      },
+    ],
+    "grammar_file": "/path/to/grammarfile"
+  }'
+```