docs: add Frontend guides (CopilotKit + AG-UI) (#6686)

* docs: add Frontend guides section (CopilotKit + AG-UI)

Add a Frontend sub-group under Guides documenting how to build user
interfaces for CrewAI Crews and Flows with CopilotKit over the AG-UI
protocol. Pages: overview, generative UI, tool-based generative UI,
agentic generative UI, human-in-the-loop, shared state, frontend
actions, predictive state updates, and channels.

* docs: mirror Frontend guides into v1.15.5 (Latest)

Also register the Frontend sub-group and pages under the default
v1.15.5 version so the section is visible without switching to Edge.

* docs(frontend): address audit — correct APIs and claims

- Use useRenderTool for display-only tool rendering (was useFrontendTool)
- Correct state 'auto-streams' claims: snapshot at step boundaries,
  document copilotkit_emit_state for mid-step progress
- Fix setState usage to spread full state (replace, not merge)
- Add tool description to the frontend-action example
- Rewrite Channels with the real @copilotkit/channels createBot API
  (Slack + Discord adapters); drop unsupported platform claims
- Note self-hosted vs managed CopilotKit paths and pin package versions

* docs(frontend): remove versions callout from overview

* docs(frontend): drop package-generation framing from emit_state note

* docs(frontend): add generative UI spectrum (A2UI, reasoning) + Conversational Flows

Rewrite generative-ui as the controlled/declarative/open-ended spectrum;
add A2UI (declarative), Reasoning (controlled), and a Conversational Flows
page; add a backend-tools section to tool-based; note the three execution
shapes in the overview.

* docs(frontend): address review — edge-only, attribute access, safe defaults

Remove the docs/v1.15.5 mirror (versioned snapshots are cut from edge by
the release tooling; the docs-snapshots CI guard rejects manual docs/v*
writes). Use attribute access on the LiteLLM message in shared-state,
guard setState against undefined agent/recipe, and use
Field(default_factory=list) for the agent-state list.

docs/docs.json

@@ -118,6 +118,24 @@
                           "edge/en/guides/flows/inputs-id-deprecation"
                         ]
                       },
+                      {
+                        "group": "Frontend",
+                        "icon": "browser",
+                        "pages": [
+                          "edge/en/guides/frontend/overview",
+                          "edge/en/guides/frontend/conversational-flows",
+                          "edge/en/guides/frontend/generative-ui",
+                          "edge/en/guides/frontend/tool-based-generative-ui",
+                          "edge/en/guides/frontend/agentic-generative-ui",
+                          "edge/en/guides/frontend/reasoning",
+                          "edge/en/guides/frontend/a2ui",
+                          "edge/en/guides/frontend/human-in-the-loop",
+                          "edge/en/guides/frontend/shared-state",
+                          "edge/en/guides/frontend/frontend-actions",
+                          "edge/en/guides/frontend/predictive-state-updates",
+                          "edge/en/guides/frontend/channels"
+                        ]
+                      },
                       {
                         "group": "Tools",
                         "icon": "wrench",