github: Add instructions for Copilot

[SeppoTakalo]

Add instructions for Copilot AI agent.
Also include clang-format rules from Zephyr to help tools.

[Copilot]

Pull request overview

Adds GitHub Copilot repository instructions and per-path instruction files, and introduces a Zephyr-derived .clang-format configuration to standardize formatting behavior for tooling in this repo.

Changes:

• Add documentation-specific and C-code-specific instruction files under .github/instructions/.
• Add a comprehensive .github/copilot-instructions.md describing repo purpose, build/test commands, and conventions.
• Add a Zephyr RTOS .clang-format configuration at the repo root.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
.github/instructions/DOC.instructions.md	Adds documentation (RST + diagrams) style guidance for doc/**.
.github/instructions/C-CODE.instructions.md	Adds embedded C guidance intended to apply to *.c/*.h.
.github/copilot-instructions.md	Adds repo-wide Copilot instructions for build, tests, layout, and conventions.
.clang-format	Adds Zephyr RTOS clang-format rules (100-col, tab-indented, Linux brace style, etc.).

Comments suppressed due to low confidence (2)

.github/instructions/C-CODE.instructions.md:29

• This link points to nrf/doc/..., but the repository doesn't contain an nrf/ directory, so the link will be broken in-repo. Consider linking to an external URL or to a path that exists in this repo/workspace (or clarify that this is a west workspace path, not a repo-relative link).

### NCS Coding Standards
- Follow contribution guidelines: [contributions.rst](nrf/doc/nrf/dev_model_and_contributions/contributions.rst)
- Follow Zephyr coding guidelines: [index.rst](zephyr/doc/contribute/index.rst)
- Reference additional guidelines in:
  - `nrf/doc/nrf/dev_model_and_contributions`
  - `zephyr/doc/contribute`

.github/instructions/DOC.instructions.md:113

• "stylised" is British spelling; the document specifies American English. Please change to "stylized".

### Connectors and arrows

- Use simple open or filled arrowheads. Avoid decorative, doubled, or overly stylised arrow ends.
- Arrowhead size: medium as a default; scale down proportionally if shapes are small.
- Place connector label text above the connector line.

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