feat(serial_uart): adds Serial (UART) documentation

[SuGlider]

Description of Change

Adds Hardware Serial documentation, mainly for the additional API on top of the mainstream Arduino API.

Test Scenarios

CI Only

Related links

N/A

[github-actions]

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[Copilot]

Pull Request Overview

This PR adds comprehensive documentation for the Serial (UART) API in the ESP32 Arduino Core. The documentation covers all aspects of UART communication including initialization, data transmission/reception, configuration options, hardware flow control, RS485 support, and event callbacks.

Key changes:

• Complete API reference for the HardwareSerial class with all methods documented
• Detailed explanation of UART availability across different ESP32 SoCs (including HP and LP UARTs)
• Configuration constants and examples for various use cases (baud rate detection, onReceive callbacks, RS485)

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[Copilot]

Pull Request Overview

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