fix: add .ipynb extension to instances tutorial links

Lychee linkcheck runs against source files, not built site URLs.
Links to notebooks need the .ipynb extension to be resolvable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/explanation/whats-new-22.md

@@ -202,6 +202,6 @@ Once a Schema is created, table definitions, inserts, queries, and all other ope
 ## See Also
 
 - [Use Isolated Instances](../how-to/use-instances.md/) — Task-oriented guide
-- [Working with Instances](../tutorials/advanced/instances/) — Step-by-step tutorial
+- [Working with Instances](../tutorials/advanced/instances.ipynb/) — Step-by-step tutorial
 - [Configuration Reference](../reference/configuration.md/) — Thread-safe mode settings
 - [Configure Database](../how-to/configure-database.md/) — Connection setup

src/how-to/use-instances.md

@@ -140,5 +140,5 @@ def test_insert(test_instance):
 ## See Also
 
 - [What's New in 2.2](../explanation/whats-new-22.md/) — Feature overview and rationale
-- [Working with Instances](../tutorials/advanced/instances/) — Step-by-step tutorial
+- [Working with Instances](../tutorials/advanced/instances.ipynb/) — Step-by-step tutorial
 - [Configuration Reference](../reference/configuration.md/) — Thread-safe mode settings