docs: add SSR hydration troubleshooting guidance

[janechu]

Pull Request

📖 Description

Adds SSR and hydration troubleshooting guidance to the FAST Element v3 server-rendering docs, including version skew, marker integrity, repeat count mismatches, declarative template resolution timing, fallback/client rendering behavior, enableHydration() timing, and isPrerendered/isHydrated diagnostics.

👩‍💻 Reviewer Notes

Please focus review on accuracy against the current v3 hydration runtime behavior.

📑 Test Plan

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

✅ 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.