docs(3.x): document <f-template> host attribute propagation in SSR page

[janechu]

Pull Request

📖 Description

Adds a new "Host Attribute Propagation" section to the 3.x declarative server-rendering documentation page (sites/website/src/docs/3.x/declarative-templates/server-rendering.md) describing how attributes declared on the inner <template> of an <f-template> propagate onto the rendered host element opening tag during SSR.

The new section covers:

• The concept and a worked example (a primary-button component with tabindex, role, and class declared on its template).
• The three supported binding forms: static name="value", dynamic name="{{expression}}", and boolean ?name="{{expression}}", including which values get stripped.
• The client-only attribute skip list (@event, :property, f-ref, f-slotted, f-children).
• The "author wins on conflict" dedupe rule (including how ?name dedupes against the bare name).

🎫 Issues

This documents the build-time renderer behavior implemented on main in microsoft/fast#7521. The feature will be picked up by the next main → releases/fast-element-v3 sync; this docs PR can land independently so that the website reflects the upcoming behavior.

👩‍💻 Reviewer Notes

• Docs-only change — no source, test, or build configuration is touched.
• The new section is placed between State Propagation and Using @microsoft/fast-build, keeping the conceptual flow (rendering contract → hydration flow → state in → host attrs out → tooling).
• Markdown is wrapped inside the existing {% raw %} Nunjucks block so that {{ }} examples render literally on the site.

📑 Test Plan

• No test changes are required. Verified the markdown edits render correctly by inspection (no broken table syntax, no unescaped Nunjucks/Liquid braces outside {% raw %}).
• Suggested smoke test for reviewer: npm run build -w @microsoft/fast-site (or the website's local dev preview) and open the rendered server-rendering page to confirm the new section displays as expected.

✅ 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.

⏭ Next Steps

When the next main → releases/fast-element-v3 sync lands, this docs page will accurately describe the behavior shipping in the v3 build pipeline. No further docs work is anticipated for this feature on the v3 track.