[skip changelog] Document sketch.always_export_binaries configuration key (#1215)

The existence and meaning of this configuration key was previously undocumented.

Author: per1234

docs/configuration.md

@@ -21,6 +21,9 @@
 - `metrics` - settings related to the collection of data used for continued improvement of Arduino CLI.
   - `addr` - TCP port used for metrics communication.
   - `enabled` - controls the use of metrics.
+- `sketch` - configuration options relating to [Arduino sketches][sketch specification].
+  - `always_export_binaries` - set to `true` to make [`arduino-cli compile`][arduino-cli compile] always save binaries
+    to the sketch folder. This is the equivalent of using the [`--export-binaries`][arduino-cli compile options] flag.
 
 ## Configuration methods
 
@@ -131,6 +134,9 @@ additional_urls = [ "https://downloads.arduino.cc/packages/package_staging_index
 [grpc]: https://grpc.io
 [sketchbook directory]: sketch-specification.md#sketchbook
 [arduino cli lib install]: commands/arduino-cli_lib_install.md
+[sketch specification]: sketch-specification.md
+[arduino-cli compile]: commands/arduino-cli_compile.md
+[arduino-cli compile options]: commands/arduino-cli_compile.md#options
 [arduino-cli config dump]: commands/arduino-cli_config_dump.md
 [arduino cli command reference]: commands/arduino-cli.md
 [arduino-cli global flags]: commands/arduino-cli_config.md#options-inherited-from-parent-commands