docs: add English Plugin Development Book

[charantejguniganti]

🚀 What this PR does

This PR introduces a comprehensive, structured 5-chapter “Plugin Development Book” in English to the official documentation. It replaces the existing placeholder that redirects users to a Japanese Zenn resource, providing a fully native and accessible learning path for English-speaking contributors.

🎯 Why this is needed

The current documentation creates friction by relying on external, non-English resources that require browser translation. This negatively impacts onboarding, especially for new contributors such as LFX participants.

This PR eliminates that barrier by delivering a clear, hands-on, end-to-end guide aligned with the latest PipeCD v1 SDK, enabling contributors to get started quickly and confidently.

🔧 What’s included

• A complete 5-chapter tutorial series covering plugin development
• Step-by-step guidance with practical examples
• Content aligned with the latest SDK and best practices
• Improved onboarding experience for global contributors

🧩 Issue(s) addressed

Fixes #6736

👤 User-facing changes

Yes — this PR adds new documentation sections and significantly improves the developer onboarding experience.

I saw PR #6737 is also addressing this area. This PR (#6736) provides the full content for the chapters rather than just a preview, which directly fulfills the structured learning path proposed in my original issue

[charantejguniganti]

Hi @khanhtc1202

I’ve opened a PR for this could you please take a look when you have some time? I’d really appreciate your feedback on whether this approach aligns with the project direction.

Happy to make any changes based on your suggestions.

Thanks!

[Ayushmore1214]

@charantejguniganti your DCO is failing plz sign it off, and lets discuss this in upcoming community meet

[Ayushmore1214]

Hey @charantejguniganti, tone of this PR and the docs feels AI-generated. We want PipeCD docs to be written by humans for humans, so if there's no further activity here we might have to close this one!

[rahulshendre]

The plugin book/documentation work is planned under the LFX mentorship.

Also, as @Ayushmore1214 mentioned, please avoid excessive AI-generated content. Using AI tools is completely fine, but contributors should fully understand and verify the code/content they are submitting.

Closing for now.