Fix docs and add migration guide

CHANGELOG.md

@@ -71,6 +71,7 @@ Until here -->
 
 ## [3.0.0] - 2025-10-13
 **Summary**: This release introduces new model dimensions (periods and scenarios) for multi-period investments and stochastic modeling, along with a redesigned effect sharing system and enhanced I/O capabilities.
+For detailed migration instructions, see the [Migration Guide](https://flixopt.github.io/flixopt/user-guide/migration-guide-v3/).
 
 ### ✨ Added
 

docs/user-guide/mathematical-notation/index.md

@@ -3,7 +3,7 @@
 
 This section provides the **mathematical formulations** underlying FlixOpt's optimization models. It is intended as **reference documentation** for users who want to understand the mathematical details behind the high-level FlixOpt API described in the [FlixOpt Concepts](../index.md) guide.
 
-**For typical usage**, refer to the [FlixOpt Concepts](../index.md) guide, [Examples](../../examples/), and [API Reference](../../api-reference/) - you don't need to understand these mathematical formulations to use FlixOpt effectively.
+**For typical usage**, refer to the [FlixOpt Concepts](../index.md) guide, [Examples](../../examples/index.md), and [API Reference](../../api-reference/index.md) - you don't need to understand these mathematical formulations to use FlixOpt effectively.
 
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