fix(docs): make mdBook SUMMARY parse — part titles, not a text list item

The first CI book build (run 26030811774) failed: `- Tutorials` was a
plain-text list item with nested children, which mdbook rejects
("nested chapters' parent must be a hyperlink"). Latent until now
because the book was never built in CI. Use mdbook part-title headers
(Tutorials / Reference) instead. Verified with mdbook v0.4.40:
builds clean, all tutorial + reference pages generated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mahabubul470

docs/book/src/SUMMARY.md

@@ -3,11 +3,16 @@
 - [Introduction](./intro.md)
 - [Installation](./install.md)
 - [Concepts](./concepts.md)
-- Tutorials
-  - [Migrating from Git to gpp](./tut-migrate.md)
-  - [Setting up Graphex for your project](./tut-graphex.md)
-  - [Connecting AI agents via MCP](./tut-mcp.md)
-  - [Compliance for regulated industries](./tut-compliance.md)
-  - [Using gpp with GitHub](./tut-github.md)
-  - [Setting up a relay node](./tut-relay.md)
+
+# Tutorials
+
+- [Migrating from Git to gpp](./tut-migrate.md)
+- [Setting up Graphex for your project](./tut-graphex.md)
+- [Connecting AI agents via MCP](./tut-mcp.md)
+- [Compliance for regulated industries](./tut-compliance.md)
+- [Using gpp with GitHub](./tut-github.md)
+- [Setting up a relay node](./tut-relay.md)
+
+# Reference
+
 - [Command reference](./cli.md)