docs: clarify FAST Element v3 browser feature support

[janechu]

Pull Request

📖 Description

Updates the FAST Element v3 browser support page to clarify support by feature:

• distinguishes the baseline client-rendered runtime from optional browser features
• documents fallback behavior for Shadow DOM configuration, adopted stylesheets, Trusted Types, globalThis, and SSR/hydration
• adds SSR/hydration notes for Declarative Shadow DOM requirements

👩‍💻 Reviewer Notes

This is documentation-only and does not change runtime behavior.

Compatibility references used for the support guidance:

• https://caniuse.com/custom-elementsv1
• https://caniuse.com/shadowdomv1
• https://caniuse.com/declarative-shadow-dom
• https://developer.mozilla.org/en-US/docs/Web/API/Document/adoptedStyleSheets
• https://developer.mozilla.org/en-US/docs/Web/API/Trusted_Types_API
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis

📑 Test Plan

• npm run build -w @microsoft/fast-site
• npm run format:check attempted; failed on existing changed-since-default formatting diffs outside this PR

✅ 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.