Sync Grok docs to upstream API surface (videos + tasks)

[Copilot]

Upstream Docs added Grok video generation and task-query APIs, while this repo’s grok/ docs only covered chat completions. This PR aligns local Grok documentation with the updated upstream OpenAPI/guide content.

• Scope alignment (affected services)

  • Updated grok/ docs to reflect new upstream endpoints:
    • POST /grok/videos
    • POST /grok/tasks
  • No local dreamina/ directory exists in this monorepo clone, so no dreamina files were changed.

• README API index updates

  • Extended grok/README.md API table to include Videos and Tasks docs.
  • Updated service summary/keywords to reflect Grok video + task capabilities.

• New integration guides added

  • grok/docs/grok_videos_generation_api_integration_guide.md
    • Model modes, request schema, async callback usage, task polling linkage, error semantics.
  • grok/docs/grok_tasks_api_integration_guide.md
    • Single/batch task retrieval patterns and response structure.

| [Grok Videos Generation API](https://platform.acedata.cloud/documents/grok-videos) | `/grok/videos` | [Grok Videos Generation API Integration Guide](docs/grok_videos_generation_api_integration_guide.md) |
| [Grok Tasks API](https://platform.acedata.cloud/documents/grok-tasks) | `/grok/tasks` | [Grok Tasks API Integration Guide](docs/grok_tasks_api_integration_guide.md) |