doc: contribute: Add multiline property style guide

    definitions.

  * Don't insert empty lines before a dedenting ``};``.

  * Insert a single empty line to separate nodes at the same hierarchy level.

  * When splitting long property values across multiple lines, specify the first

    value on the same line as the opening bracket (``<`` or ``[``). Place the

    closing bracket and semicolon (``>;`` or ``];``) on the same line after

    the final value of the property.

Examples:

.. literalinclude:: style-example.dts

  :language: devicetree

  :start-after: start-after-here

/*

 * SPDX-FileCopyrightText: Copyright The Zephyr Project Contributors

 *

 * SPDX-License-Identifier: Apache-2.0

 *

 * Demonstration of permitted and disallowed coding style for devicetree files.

 */

/* start-after-here */

/dts-v1/;

/ {

	/* Node labels use the "_" separator, but node names use "-". */

	psa_rng: psa-rng {

		compatible = "zephyr,psa-crypto-rng";

		status = "okay";

	};

	ramdisk0 {

		compatible = "zephyr,ram-disk";

		/* Property names use the "-" separator. */

		disk-name = "RAM";

		sector-size = <512>;

		sector-count = <128>;

	};

	periph0 {

		/* Wrap array values to conform with line lengths. Indent

		 * continuation so that values align.

		 */

		pinctrl-0 = <&periph0_siga_px0_default &periph0_sigb_py7_default

			     &periph0_sigc_pz1_default>;

		pinctrl-names = "default";

	};

	mipi-dbi {

		compatible = "zephyr,mipi-dbi-spi";

		lcd0: lcd@0 {

			compatible = "sitronix,st7735r";

			/* Split array values across multiple lines to help readability. */

			gamctrp1 = [10 0E 02 03

				    0E 07 02 07

				    0A 12 27 37

				    00 0D 0E 10];

			gamctrn1 = [10 0E 03 03

				    0F 06 02 08

				    0A 13 26 36

				    00 0D 0E 10];

		};

	};

};