chore: add a guide for Header

Author: skokado

docs/docs/guides/input/header.md

@@ -0,0 +1,46 @@
+# Request Header
+
+Django Ninja provides a convenient `Header()` parameter function to extract values from HTTP request headers in your API endpoints.
+
+## Usage Patterns
+
+There are two main ways to work with headers:
+
+### 1. Custom Header Name
+
+!!! note
+    Use the `alias` parameter to specify the exact HTTP header name (with hyphens)
+    Do NOT use underscores in the parameter name, as Python identifiers cannot contain hyphens.
+
+- Python 3.6+ non-Annotated
+
+```python hl_lines="5"
+{!./src/tutorial/header/code01.py!}
+```
+
+- Python 3.9+ Annotated
+
+```python hl_lines="5 9"
+{!./src/tutorial/header/code02.py!}
+```
+
+This pattern is ideal for:
+- Custom headers (e.g., `X-API-Key`, `X-Request-ID`)
+- Non-standard header names
+- When you want explicit control over header mapping
+
+### 2. Well-Known Headers
+
+For common HTTP headers, you can use snake_case parameter names without an alias:
+
+```python hl_lines="5 11 19 23"
+{!./src/tutorial/header/code03.py!}
+```
+
+Common implicit headers include:
+
+- `user_agent` → `User-Agent`
+- `content_length` → `Content-Length`
+- `content_type` → `Content-Type`
+- `authorization` → `Authorization`
+- etc.

docs/mkdocs.yml

@@ -55,6 +55,7 @@ nav:
           - guides/input/path-params.md
           - guides/input/query-params.md
           - guides/input/body.md
+          - guides/input/header.md
           - guides/input/form-params.md
           - guides/input/file-params.md
           - guides/input/request-parsers.md

docs/src/tutorial/header/code01.py

@@ -0,0 +1,6 @@
+from ninja import Header
+
+
+@api.get("/events")
+def events(request, custom_header: str = Header(alias="X-Custom-Header")):
+    return {"received": custom_header}

docs/src/tutorial/header/code02.py

@@ -0,0 +1,10 @@
+from typing import Annotated
+from ninja import Header
+
+
+CustomHeader = Annotated[str, Header(alias="X-Custom-Header")]
+
+
+@api.get("/events")
+def events(request, custom_header: CustomHeader):
+    return {"received": custom_header}

docs/src/tutorial/header/code03.py

@@ -0,0 +1,24 @@
+from ninja import Header
+
+
+@api.get("/events")
+def events(request, user_agent: str | None = Header(None)):
+    return {"user_agent": user_agent}
+
+
+# Using alias instead of argument name
+@api.get("/events")
+def events(request, ua: str | None = Header(None, alias="User-Agent")):
+    return {"user_agent": ua}
+
+
+# Python 3.9+ Annotated
+from typing import Annotated
+
+
+UserAgent = Annotated[str | None, Header(None)]
+
+
+@api.get("/events")
+def events(request, user_agent: UserAgent):
+    return {"user_agent": user_agent}