[EPIC] Improve and Expand Contributing Section in Documentation

The current Contributing section on pipecd.dev only links to CONTRIBUTING.md and a basic architecture page. While helpful, it lacks the depth and guidance needed to support both new contributors and users.

Goals

• Make the section more welcoming for first-time contributors.
• Provide clear pathways for different contribution types (docs, code, design, translations, etc.).
• Include architecture context, but also practical steps for setup, workflow, and PR process.
• Highlight community channels, issue labels, and where to seek help

Discussion Prompt:

What content, guidance, or structure would make this section more useful? Share your ideas below.