Add a developer documentation section to the LinearAlgebra docs (#5…

…6324)

Functions that are meant for package developers may go here, instead of
the main section that is primarily for users.

stdlib/LinearAlgebra/docs/src/index.md

@@ -891,6 +891,12 @@ LinearAlgebra.LAPACK.trsyl!
 LinearAlgebra.LAPACK.hseqr!
 ```
 
+## Developer Documentation
+
+```@docs
+LinearAlgebra.matprod_dest
+```
+
 ```@meta
 DocTestSetup = nothing
 ```