Documentation for nonlinear Picard and Newton solvers

Add Nonlinear Solver Section to Heat_Transport_BHE

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web/content/docs/processes/heat-transport/HEAT_TRANSPORT_BHE/index.md

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 </refrigerant>
 ```
 
+## Available Nonlinear Solvers
+
+In the `HEAT_TRANSPORT_BHE` process, both Picard and Newton nonlinear solvers are supported.
+With the Picard solver, the linear equation system is iterated until the change in the solution vector is small enough (typically less than 1E-6).
+With the Newton approach, the global Jacobian matrix and residual vector are evaluated in each iteration, in order to solve for the change of solution vector in each iteration.
+The solved change vector is then further used to update the global solution vector.
+Typically, after 5 to 8 iterations, the Newton approach leads to a globally converged solution, i.e. the norm of the residual vector is less than certain tolerance (e.g. relative tolerance less than 1E-10).
+When configuring an OGS project with the `HEAT_TRANSPORT_BHE` process, Picard iterations will be sufficient when the time step size is kept small (tens of seconds).
+However, when the time step is large (hours or days), Newton iterations may reach convergence much faster than the Picard approach.
+
+For more information regarding how to configure the Picard and Newton nonlinear solvers, please also refer to the section in [Project File - Building Blocks](/docs/userguide/blocks/nonlinear_solvers)
+
 ## References
 
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web/content/docs/userguide/blocks/nonlinear_solvers.md

@@ -29,13 +29,13 @@ Following non-linear solvers are available in OpenGeoSys:
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 The nonlinear solver of "Newton" type is an implementation of the Newton-Raphson method.
-The basic definition of the non-linear solver follows this template:
+The basic definition of the non-linear solver with "Newton" follows this template:
 
 ```xml
 <nonlinear_solver>
     <name>basic_newton</name>
     <type>Newton</type>
-    <max_iter>100</max_iter>
+    <max_iter>10</max_iter>
     <linear_solver>linear_solver</linear_solver>
 </nonlinear_solver>
 ```
@@ -43,3 +43,15 @@ The basic definition of the non-linear solver follows this template:
 ## Picard
 
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+
+The nonlinear solver of "Picard" type is an implementation of the Picard-Iteration method.
+The basic definition of the non-linear solver with "Picard" follows this template:
+
+```xml
+<nonlinear_solver>
+    <name>basic_picard</name>
+    <type>Picard</type>
+    <max_iter>100</max_iter>
+    <linear_solver>linear_solver</linear_solver>
+</nonlinear_solver>
+```