PipeCD - Enhanced Contributing Guidelines

Expanded PipeCD's contribution docs on community, development setup, and the PR process, to help new contributors get started.

I expanded PipeCD’s contributing docs to make getting started clearer for first-time contributors.

The update added practical guidance on where to ask questions, how to set up a local dev environment, and how to structure PRs so they get reviewed faster.

I made these changes across multiple docs files and kept the wording and structure the same in both docs-dev and docs-v1.0.x.

This made the contribution steps clearer. It was merged after a few rounds of maintainer feedback.

Links: PR #6399 • Issue #6124