Docs: Add CI overview document explaining GitHub workflows

[Ayushmore1214]

What would you like to be added: A documentation page that explains PipeCD’s GitHub Actions CI setup, including what each workflow does, when it runs, and how contributors can understand or debug CI failures.

Why is this needed: Having a clear CI document would make it easier for new contributors to understand the CI system, debug issues on their own, and contribute with more confidence, while also reducing repeated questions for maintainers.

[Ayushmore1214]

@eeshaanSA should I go ahead and make a pr ?

[eeshaanSA]

cc/

@khanhtc1202
@Warashi

[khanhtc1202]

Hi @Ayushmore1214 thanks for the idea, yes you can go ahead with you idea, that looks neat 👍

[Ayushmore1214]

Thanks for approval will raise a pr soon !