[docs](function) Add ST_IsClosed documentation

docs/sql-manual/sql-functions/scalar-functions/spatial-functions/st-isclosed.md

@@ -0,0 +1,95 @@
+---
+{
+    "title": "ST_ISCLOSED",
+    "language": "en",
+    "description": "Determines whether a LineString is closed by comparing its first and last points."
+}
+---
+
+## Description
+
+Determines whether a `LINESTRING` is closed. A `LINESTRING` is closed when its first and last points are exactly equal. This function does not use a distance tolerance.
+
+## Syntax
+
+```sql
+ST_ISCLOSED( <shape> )
+```
+
+## Parameters
+
+| Parameter | Description |
+| :--- | :--- |
+| `<shape>` | The input geometry, of type GEOMETRY or VARCHAR (in WKT format) that can be converted to GEOMETRY. |
+
+## Return Value
+
+Returns a BOOLEAN value:
+
+- `true` if the input is a valid `LINESTRING` whose first and last points are exactly equal.
+- `false` if the input is a valid `LINESTRING` whose first and last points are different.
+- `NULL` if the input is `NULL`, is not a `LINESTRING`, or cannot be decoded as a valid geometry.
+
+In SQL output, `true` is displayed as `1` and `false` as `0`.
+
+## Example
+
+**Closed LineString**
+
+```sql
+SELECT ST_IsClosed(
+    ST_GeometryFromText('LINESTRING(0 0, 1 1, 0 0)')
+) AS is_closed;
+```
+
+```text
++-----------+
+| is_closed |
++-----------+
+|         1 |
++-----------+
+```
+
+**Open LineString**
+
+```sql
+SELECT ST_IsClosed(
+    ST_GeometryFromText('LINESTRING(0 0, 1 1)')
+) AS is_closed;
+```
+
+```text
++-----------+
+| is_closed |
++-----------+
+|         0 |
++-----------+
+```
+
+**NULL Input**
+
+```sql
+SELECT ST_IsClosed(NULL) AS is_closed;
+```
+
+```text
++-----------+
+| is_closed |
++-----------+
+|      NULL |
++-----------+
+```
+
+**Non-LineString Input**
+
+```sql
+SELECT ST_IsClosed(ST_Point(0, 0)) AS is_closed;
+```
+
+```text
++-----------+
+| is_closed |
++-----------+
+|      NULL |
++-----------+
+```