docs: document post-#488 RPC lifecycle on the server-actions docs page

[vivek7405]

Problem

The docs site Server Actions page (docs/app/docs/server-actions/page.ts) is the canonical reference for server actions, but it was never updated for the post-#488 RPC lifecycle features. A reader who lands there has no path to per-action middleware, cancellation, or streaming results. Those features shipped their docs only on the Data Fetching page (docs/app/docs/data-fetching/page.ts), so the canonical reference is incomplete.

This is the same surface-drift pattern that produced the #488 gap: the feature is fully documented in AGENTS.md and agent-docs/, but the docs site lags.

Implementation notes (for the implementing agent)

• Where to edit: docs/app/docs/server-actions/page.ts. Add a section after "The route() Adapter" and before the ActionResult envelope, covering:
  • Per-action middleware (feat: composable per-action middleware with typed context #490): export const middleware = [...] and actionContext() from @webjsdev/server.
  • Cancellation (feat: AbortSignal cancellation for RPC, wired to async-render supersede #492): actionSignal() plus the superseded-async-render auto-abort.
  • Streaming results (feat: streaming RPC results (return a ReadableStream/AsyncIterable from an action) #489): a one-line cross-link to the Data Fetching streaming section is enough (that page already covers it fully).
• Source of truth to copy from: docs/app/docs/data-fetching/page.ts (around L90 to L93), AGENTS.md around L264, agent-docs/advanced.md (the streaming RPC section).
• The segment-middleware page (docs/app/docs/middleware/page.ts around L201) already cross-references per-action middleware, so only the server-actions page itself is missing it.

Acceptance criteria

• Server Actions docs page documents export const middleware + actionContext().
• Server Actions docs page documents actionSignal() cancellation.
• Server Actions docs page cross-links the streaming-results section.
• Prose matches the behaviour described in AGENTS.md and the data-fetching page.