PipeCD - Plugin Contribution Guide

Wrote a full guide for contributing to PipeCD's plugins, covering how the architecture works and good development practices.

I wrote and merged PipeCD’s plugin contribution guide, so new contributors can start building plugins without guesswork.

The guide explains how plugins connect to PipeCD core, the lifecycle and rules a plugin must follow, and practical ways to set up and test plugins safely.

I also matched the guide to the wider docs structure updates in docs-dev and docs-v1.0.x, so the plugin docs fit with the rest of the contribution docs.

This work came from issue #6124 and shipped after several rounds of maintainer review.

Links: PR #6398 • Issue #6124