[skip changelog] Document sketch.always_export_binaries configuration key

[per1234]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

docs update

• What is the current behavior?

The existence and meaning of the sketch.always_export_binaries configuration key (#1042) is undocumented.

• What is the new behavior?

The sketch.always_export_binaries configuration key is documented in the Arduino CLI Configuration page of the documentation website:
https://arduino.github.io/arduino-cli/dev/configuration/

• Does this PR introduce a breaking change, and is
  titled accordingly?

No

[ubidefeo]

@per1234 , should the system ENV variable also be mentioned somewhere?

[per1234]

Configuration via environment variables is documented here:
https://arduino.github.io/arduino-cli/dev/configuration/#environment-variables
This explains how the environment variable name is derived from the configuration key name. I think this is better than maintaining parallel documentation for the specific environment variable names.

[ubidefeo]

@per1234 agreed.
Thank you :)

[ubidefeo]

LGTM