Restructure API reference documentation

Co-Authored-By: Claude Code (claude-sonnet-4-6) <noreply@anthropic.com>

Author: Copilot

docs/src/70-reference.md

@@ -4,6 +4,17 @@
 Pages = ["70-reference.md"]
 ```
 
+## Public API
+
+The functions and types below form the supported public interface of
+TulipaEnergyModel.jl.
+
 ```@autodocs
 Modules = [TulipaEnergyModel]
+Private = false
 ```
+
+## Internal implementation details
+
+Internal helper functions (such as names starting with `_`) are not part of
+the public API and are intentionally omitted from this page.