Move internal APIs to Internal Details section

- Moved structural_transformation.md and bipartite_graph.md from API/ to internals/
- Created proper Internal Details section with overview and warnings
- Added clear warnings that these are internal APIs not for public use
- Updated pages.jl to reflect new documentation structure

These functions are implementation details and should not be relied upon by users.

Generated with Claude Code (https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

docs/pages.jl

@@ -32,8 +32,6 @@ pages = [
         "API/variables.md",
         "API/model_building.md",
         "API/problems.md",
-        "API/structural_transformation.md",
-        "API/bipartite_graph.md",
         "API/dynamic_opt.md",
         "API/codegen.md",
         "API/PDESystem.md"],
@@ -49,5 +47,8 @@ pages = [
         "basics/Precompilation.md",
         "basics/FAQ.md"],
     "comparison.md",
-    "internals.md"
+    "Internal Details" => Any[
+        "internals.md",
+        "internals/structural_transformation.md",
+        "internals/bipartite_graph.md"]
 ]

docs/src/internals.md

@@ -1,4 +1,16 @@
 # Internal Details
 
-This is a page for detailing some of the inner workings to help future
-contributors to the library.
+This section documents the internal implementation details of ModelingToolkit. These APIs are not considered stable and may change without notice in non-breaking releases. They are documented here to help future contributors understand the library's inner workings.
+
+## Overview
+
+ModelingToolkit's internal architecture consists of several key components:
+
+- **Structural Transformation**: Algorithms for transforming equation systems, including index reduction, tearing, and algebraic simplification
+- **Bipartite Graphs**: Graph representations used to analyze relationships between equations and variables
+- **System Structure**: Internal representations of system state and transformations
+
+These components work together to enable ModelingToolkit's symbolic manipulation and code generation capabilities.
+
+!!! warning
+    The functions and types documented in this section are internal implementation details. Users should not rely on these APIs as they may change or be removed without deprecation warnings.

docs/src/API/bipartite_graph.md renamed to docs/src/internals/bipartite_graph.md

@@ -1,5 +1,8 @@
 # Bipartite Graphs
 
+!!! warning "Internal API"
+    The functions documented on this page are internal implementation details of ModelingToolkit. They are not part of the public API and may change or be removed without notice in non-breaking releases. This documentation is provided to help contributors understand the codebase.
+
 ModelingToolkit uses bipartite graphs to represent relationships between equations and variables in systems. These functions provide tools for working with and analyzing these graphs.
 
 ## Graph Types

docs/src/API/structural_transformation.md renamed to docs/src/internals/structural_transformation.md

@@ -1,5 +1,8 @@
 # Structural Transformation
 
+!!! warning "Internal API"
+    The functions documented on this page are internal implementation details of ModelingToolkit. They are not part of the public API and may change or be removed without notice in non-breaking releases. This documentation is provided to help contributors understand the codebase.
+
 These functions are used for structural analysis and transformation of equation systems, including index reduction, tearing, and other algebraic manipulations used in the simplification process.
 
 ## Tearing and Algebraic Simplification