docs: add documentation for file export feature

Author: esadek

README.md

@@ -13,6 +13,7 @@ A command-line tool for querying databases via [ADBC](https://arrow.apache.org/a
 - **Interactive SQL shell** - Execute SQL queries with command history and intuitive navigation
 - **Syntax highlighting** - SQL queries highlighted for improved readability
 - **Formatted output** - Results displayed in clean, aligned tables with dynamic column width
+- **File export** - Export query results to JSON, CSV, or Arrow IPC files
 - **Fast and lightweight** - Built in Rust for high performance and minimal resource usage
 
 ## Installation
@@ -97,6 +98,14 @@ $ adbcli --driver duckdb --file select_example.sql
 └────┴───────┴────────────┘
 ```
 
+Execute a query and output the result to a file:
+
+```console
+$ adbcli --driver duckdb --query "SELECT 42 AS the_answer" --output result.json
+$ adbcli --driver duckdb --query "SELECT 42 AS the_answer" --output result.csv
+$ adbcli --driver duckdb --query "SELECT 42 AS the_answer" --output result.arrow
+```
+
 ## Reference
 
 ```console
@@ -114,6 +123,7 @@ Options:
       --mode <mode>          Table display style [default: utf8_compact]
       --query <query>        Execute query and exit
       --file <file>          Read and execute file and exit
+      --output <file>        Write result to file
   -h, --help                 Print help
   -V, --version              Print version
 ```

docs/index.md

@@ -17,6 +17,7 @@ A command-line tool for querying databases via [ADBC](https://arrow.apache.org/a
 - **Interactive SQL shell** - Execute SQL queries with command history and intuitive navigation
 - **Syntax highlighting** - SQL queries highlighted for improved readability
 - **Formatted output** - Results displayed in clean, aligned tables with dynamic column width
+- **File export** - Export query results to JSON, CSV, or Arrow IPC files
 - **Fast and lightweight** - Built in Rust for high performance and minimal resource usage
 
 ## Getting Started
@@ -91,3 +92,11 @@ $ adbcli --driver duckdb --file select_example.sql
 │ 2  │ Alice │ 2001-03-22 │
 └────┴───────┴────────────┘
 ```
+
+Execute a query and output the result to a file:
+
+```console
+$ adbcli --driver duckdb --query "SELECT 42 AS the_answer" --output result.json
+$ adbcli --driver duckdb --query "SELECT 42 AS the_answer" --output result.csv
+$ adbcli --driver duckdb --query "SELECT 42 AS the_answer" --output result.arrow
+```

docs/reference.md

@@ -90,6 +90,22 @@ Read and execute file and exit
 adbcli --driver duckdb --file select_example.sql
 ```
 
+## --output
+
+Write result to file
+
+```sh
+adbcli --driver duckdb --query "SELECT 42 AS the_answer" --output result.json
+```
+
+The output format is inferred from the file extension:
+
+| Extension       | Format    |
+|-----------------|-----------|
+| `.json`         | JSON      |
+| `.csv`          | CSV       |
+| `.arrow`, `.ipc`| Arrow IPC |
+
 ## --help
 
 Print the help message