chore: reorganize docs (update sql-> func)

docs/references/func.md

@@ -0,0 +1,5 @@
+# Functions
+
+Use built-in functions for data manipulation and analysis to operate on the underlying database storing the chain data. These functions are useful for operations like [`DataChain.filter`](datachain.md#datachain.lib.dc.DataChain.filter) and [`DataChain.mutate`](datachain.md#datachain.lib.dc.DataChain.mutate). Import these functions from `datachain.func`.
+
+::: datachain.func

docs/references/index.md

@@ -10,5 +10,5 @@ DataChain's API is organized into several modules:
 - [DataType](./datatype.md) - Type system and schema definitions
 - [File](./file.md) - File handling and storage operations
 - [UDF](./udf.md) - User-defined functions and transformations
-- [SQL](./sql.md) - SQL query integration
+- [Functions](./func.md) - Built-in functions for data manipulation and analysis
 - [Torch](./torch.md) - PyTorch data loading utilities

mkdocs.yml

@@ -73,7 +73,7 @@ nav:
           - File: references/file.md
           - UDF: references/udf.md
           - Torch: references/torch.md
-          - SQL: references/sql.md
+          - Functions: references/func.md
       - 🤝 Contributing: contributing.md
 
   - DataChain Website ↗: https://datachain.ai" target="_blank"