PipeCD - Documentation Quality Improvements

Fixed spelling and grammar issues across many documentation files in PipeCD, a CNCF Sandbox project, to make the docs clearer and more polished.

I cleaned up PipeCD's documentation by fixing spelling, grammar, and naming issues in 25 files.

This included fixes like manifiests → manifests, correct capitalization for terms such as Kubernetes and EventWatcher, and small grammar fixes that made sentences easier to read.

It was a simple cleanup, but it made the core docs clearer for users and contributors.

Links: PR #6361 • Issue #6357