docs(pwa): add guide for environment-specific manifests (#10458)

[yahyanaujid]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Closes #10458.

The current PWA plugin documentation does not explain how to handle multiple manifest.json files for different environments (e.g., staging vs. production). This PR adds a brief guide demonstrating how to manage environment-specific manifests using a pre-build file copy script in package.json, which is the safest and most standard approach.

Test Plan

N/A - This is a pure documentation update. Verified MDX formatting and layout locally.

Related issues/PRs

Resolves #10458

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