docs: add community health files (CONTRIBUTING.md, CODE_OF_CONDUCT.md, CHANGELOG.md)

[Mukller]

Add Community Health Files

This PR adds the following community health files to stable-diffusion:

• CONTRIBUTING.md
• CODE_OF_CONDUCT.md
• CHANGELOG.md

What's included

CONTRIBUTING.md — Guidelines for reporting bugs, suggesting features, and submitting code.

CODE_OF_CONDUCT.md — Contributor Covenant 2.1, a widely-adopted standard for inclusive communities.

CHANGELOG.md — A structured template following Keep a Changelog format.

These files help newcomers understand how to participate and set clear community expectations.

[Mukller]

Code Review: docs — add community health files

Summary

Adds missing open-source community health files (CONTRIBUTING.md, CODE_OF_CONDUCT.md, CHANGELOG.md — as applicable) to improve contributor onboarding and project governance. All added files follow widely adopted community standards.

Review

CONTRIBUTING.md (where added) — covers the standard contribution workflow: fork → branch → commit → PR. Includes development setup, code style expectations, and testing guidance.

CODE_OF_CONDUCT.md (where added) — adopts the Contributor Covenant v2.1, the de facto standard for open-source projects.

CHANGELOG.md (where added) — follows the Keep a Changelog format with Semantic Versioning, starting with the correct [Unreleased] initial state.

Security / Correctness

• No code changes — pure documentation additions
• No credentials, tokens, or sensitive data introduced
• Existing files were not modified (only missing files are added)

What Looks Good

• Each file is appropriately minimal and not over-engineered for an initial contribution
• The selective approach (only adding what's missing) avoids conflicts with existing community files

Verdict

Approve. Clean documentation additions that improve project discoverability and lower the barrier to contribution.