docs/core: align LED_BUILTIN precedence and fix DTS docs

[soburi]

Motivation

• The code and documentation were inconsistent about how LED_BUILTIN is derived when /zephyr,user/digital-pin-gpios is not present, causing the documented precedence (variant.h > /zephyr,user/builtin-led-gpios > led0) to not be respected in practice.
• Documentation examples for connector devicetree fragments contained a missing closing }; that could confuse readers and tools.
• document/variants.md is intentionally left unchanged.

Description

• Update cores/arduino/Arduino.h to consider /zephyr,user/builtin-led-gpios before falling back to the led0 alias when digital_pin_gpios is not defined, restoring the documented precedence for LED_BUILTIN.
• Fix the arduino_header DTS examples by adding the missing root closing }; in documentation/configuration-guide.md and documentation/configuration-reference.md.
• Simplify duplicated wording in the arduino_adc notes in documentation/configuration-reference.md to improve readability.

Testing

• Ran git diff --check to ensure no whitespace or index issues, which completed without errors.
• Verified Markdown code fence parity for modified docs using a small Python script that counted fences, and the counts were balanced.
• Performed quick source searches for LED/builtin-led references to confirm the new builtin_led_gpios check is present and documented behavior is aligned.

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Codex Task

[Copilot]

Pull request overview

Aligns ArduinoCore-Zephyr’s LED_BUILTIN derivation behavior with the documented precedence when /zephyr,user/digital-pin-gpios is not defined, and fixes minor DTS documentation issues that could confuse readers/tools.

Changes:

• Update cores/arduino/Arduino.h so /zephyr,user/builtin-led-gpios is considered before falling back to the led0 alias when digital_pin_gpios is absent.
• Fix DTS examples in docs by adding the missing root closing };.
• Simplify duplicated wording in the arduino_adc notes in the configuration reference.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
cores/arduino/Arduino.h	Restores documented LED_BUILTIN precedence (builtin-led-gpios before led0) in the non-digital-pin-gpios code path.
documentation/configuration-guide.md	Fixes DTS example structure by adding the missing closing }; for the root node.
documentation/configuration-reference.md	Fixes DTS example structure, and cleans up arduino_adc notes formatting/duplication.

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