docs: add ClickHouse chat agent example project page (#4195)

## What

Adds a new Example projects page: **ClickHouse chat agent** — a
`chat.agent()` that answers questions about your data by writing and
running SQL against ClickHouse Cloud via the official Node.js ClickHouse
client.

The page follows the existing example-project format (overview,
features, GitHub repo card, how-it-works with code excerpts, relevant
code links) and is registered in `docs.json` in alphabetical order.

## Note

The GitHub repo card links to
`triggerdotdev/examples/tree/main/clickhouse-chat-agent`, which lands in
a companion examples PR — merge that one first.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

docs/docs.json

@@ -526,6 +526,7 @@
               "guides/example-projects/claude-changelog-generator",
               "guides/example-projects/claude-github-wiki",
               "guides/example-projects/claude-thinking-chatbot",
+              "guides/example-projects/clickhouse-chat-agent",
               "guides/example-projects/cursor-background-agent",
               "guides/example-projects/human-in-the-loop-workflow",
               "guides/example-projects/mastra-agents-with-memory",