docs: add CONTRIBUTING.md file

Signed-off-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>

Author: gbartolini

CODE_OF_CONDUCT.md

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+# Code of Conduct
+
+The Code of Conduct for the CloudNativePG Community can be found in the
+[governance repository](https://github.com/cloudnative-pg/governance/blob/main/CODE_OF_CONDUCT.md).

CONTRIBUTING.md

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+# Contributing to CloudNativePG
+
+Thank you for your interest in contributing! 💖
+
+To ensure consistency across the project, all CloudNativePG repositories follow
+a common set of guidelines regarding code of conduct, AI usage, and
+contribution workflows.
+
+Please review the [CloudNativePG Project contributing guidelines](https://github.com/cloudnative-pg/governance/blob/main/CONTRIBUTING.md)
+before searching for issues, reporting bugs, or submitting a pull request.

GOVERNANCE.md

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+# CloudNativePG Governance
+
+Explore the governance policies for the CloudNativePG Community in the
+[dedicated repository](https://github.com/cloudnative-pg/governance/blob/main/GOVERNANCE.md).
+