Add Grok to the website and supported-providers docs

Adds the /grok SEO landing page (and its auto-generated route/sitemap
entries), the /agents directory card, and the Grok line in the ACP
catalog docs, following the Grok Build provider added to the in-app
catalog.

public-docs/supported-providers.md

@@ -41,6 +41,7 @@ Pick any of these from the in-app provider catalog. Each entry is a one-click in
 - [GitHub Copilot](https://github.com/features/copilot/cli/), GitHub's AI pair programmer via ACP.
 - [GLM Agent](https://github.com/stefandevo/glm-acp-agent), Zhipu AI's GLM coding agent.
 - [goose](https://block.github.io/goose/), Block's local open-source AI agent.
+- [Grok](https://docs.x.ai/build/overview), xAI's Grok Build agentic coding CLI.
 - [Hermes Agent](https://hermes-agent.nousresearch.com/docs/user-guide/features/acp), Nous Research's self-improving agent.
 - [Junie](https://junie.jetbrains.com/docs/junie-cli-acp.html), JetBrains' coding agent.
 - [Kilo Code](https://kilo.ai/docs/code-with-ai/platforms/cli), open-source coding agent.