Remove documentation for @Matchable internal types

- Remove AtIndex docstring as it's an internal @Matchable type
- Remove AtIndex from api.md documentation

These are internal implementation details and should not be part of
the public API documentation.

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

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

Author: ChrisRackauckas

docs/src/api.md

@@ -104,7 +104,6 @@ SymbolicUtils.SetArray
 SymbolicUtils.MakeArray
 SymbolicUtils.MakeSparseArray
 SymbolicUtils.MakeTuple
-SymbolicUtils.AtIndex
 ```
 
 ### Parallelism

src/code.jl

@@ -446,17 +446,6 @@ SetArray
 
 SetArray(inbounds, arr, elems) = SetArray(inbounds, arr, elems, false)
 
-"""
-    AtIndex(i, elem)
-
-Represent an element at a specific index in an array.
-
-This is used internally for array indexing operations in code generation.
-
-# Fields
-- `i`: The index
-- `elem`: The element at that index
-"""
 @matchable struct AtIndex <: CodegenPrimitive
     i
     elem