Fix broken PDESystem documentation link

The ModelingToolkit PDESystem docs moved from systems/PDESystem/ to
API/PDESystem/, causing a 403 linkcheck error.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

docs/src/manual/bpinns.md

@@ -2,7 +2,7 @@
 
 Using the Bayesian PINN solvers, we can solve general nonlinear PDEs, ODEs and also simultaneously perform parameter estimation on them.
 
-Note: The BPINN PDE solver also works for ODEs defined using ModelingToolkit, [ModelingToolkit.jl PDESystem documentation](https://docs.sciml.ai/ModelingToolkit/stable/systems/PDESystem/). Despite this, the ODE specific BPINN solver `BNNODE` [refer](https://docs.sciml.ai/NeuralPDE/dev/manual/ode/#NeuralPDE.BNNODE) exists and uses `NeuralPDE.ahmc_bayesian_pinn_ode` at a lower level.
+Note: The BPINN PDE solver also works for ODEs defined using ModelingToolkit, [ModelingToolkit.jl PDESystem documentation](https://docs.sciml.ai/ModelingToolkit/stable/API/PDESystem/). Despite this, the ODE specific BPINN solver `BNNODE` [refer](https://docs.sciml.ai/NeuralPDE/dev/manual/ode/#NeuralPDE.BNNODE) exists and uses `NeuralPDE.ahmc_bayesian_pinn_ode` at a lower level.
 
 # `BayesianPINN` Discretizer for PDESystems and lower level Bayesian PINN Solver calls for PDEs and ODEs.
 

docs/src/manual/pinns.md

@@ -23,7 +23,7 @@ PDEs are defined using the ModelingToolkit.jl `PDESystem`:
 Here, `eq` is the equation, `bcs` represents the boundary conditions, `param` is
 the parameter of the equation (like `[x,y]`), and `var` represents variables (like `[u]`).
 For more information, see the
-[ModelingToolkit.jl PDESystem documentation](https://docs.sciml.ai/ModelingToolkit/stable/systems/PDESystem/).
+[ModelingToolkit.jl PDESystem documentation](https://docs.sciml.ai/ModelingToolkit/stable/API/PDESystem/).
 
 ## The `PhysicsInformedNN` Discretizer