docs: add DOM policy and Trusted Types guide

[janechu]

Pull Request

📖 Description

Adds a FAST Element 3.x guide for configuring DOMPolicy and Trusted Types. The guide documents DOM.setPolicy(), DOMPolicy.create(), Trusted Types CSP considerations, default policy limitations, XSS guidance, and policy examples, with links from relevant advanced documentation.

📑 Test Plan

• npm run build -w @microsoft/fast-site
• npm run format:check
• npm run biome:ci
• npm run checkchange

Note: npm run biome:check exits 1 with no diagnostics for this docs-only change because the changed files are Markdown and Biome ignores them.

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.