Docs: Add Docs for New elements

[spuppo-mux]

Document the public API for the work done in parent issue

[github-actions]

Triage notes

Title: Added the Docs: prefix.

Labels: Added docs, spf, media, api.

Not a duplicate: The existing site/src/content/docs/reference/background-video.mdx page (from #727/#1253) documents the unrelated plain <background-video> custom element (packages/html/src/media/background-video), which wraps HTML5 <video> and doesn't use SPF. This issue is about the newer SPF engine (createBackgroundVideoEngine / BackgroundVideoMediaElement in packages/spf/src/playback/engines/background-video, shipped via #1586), which has no docs yet.

Scope to confirm: Parent epic #873 is still open — only Phase 1 (SPF composition + adapter) has landed; the actual <mux-background-video> Video.js component (Phase 5) doesn't exist yet. Worth confirming whether this issue is scoped to documenting the SPF engine/adapter API now (following the existing packages/spf/docs/hls-engine.md walkthrough convention — a background-video-engine.md would fit naturally) versus waiting to document a future user-facing component once #873 Phase 5 lands.

Planning recommendations:

• Related: Feature: Create mux-background-video Element #873 (parent epic, open) and the design doc at internal/design/spf/use-cases/background-video.md.
• Given the scope-clarification above, holding off on a milestone/roadmap addition seems reasonable until it's confirmed whether this targets SPF-internal docs (can proceed now) or the future component's docs (blocked on Feature: Create mux-background-video Element #873 Phase 5).