Calculate expression added to modeler tools (#8550)

docs/user_manual/processing_algs/qgis/modelertools.rst

@@ -14,6 +14,66 @@ Modeler tools
       :class: toc_columns
 
 
+.. _qgiscalculateexpression:
+
+Calculate expression 
+--------------------
+|334|
+
+It calculates the result of a QGIS expression and eliminates 
+the need to use the same expression multiple times throughout 
+a model when the same result is required more than once. 
+Additionally, it enables use cases that would otherwise not be
+possible. For instance, you can generate a timestamp value once
+and use it multiple times within the model, if the timestamp 
+were recalculated every time, the values would vary during the
+model's runtime.
+
+Parameters
+..........
+
+.. list-table::
+   :header-rows: 1
+   :widths: 20 20 20 40
+
+   * - Label
+     - Name
+     - Type
+     - Description
+   * - **Input**
+     - ``INPUT``
+     - [expression]
+     - Expression to calculate 
+
+Outputs
+.......
+
+.. list-table::
+   :header-rows: 1
+   :widths: 20 20 20 40
+
+   * - Label
+     - Name
+     - Type
+     - Description
+   * - **Value**
+     - ``OUTPUT``
+     - [Result Value]
+     - Calculated result value, the data type of the
+       output will vary based on the specific expression used 
+       in the algorithm.
+
+Python code
+...........
+
+**Algorithm ID**: ``native:calculateexpression``
+
+.. include:: ../algs_include.rst
+  :start-after: **algorithm_code_section**
+  :end-before: **end_algorithm_code_section**
+
+
+
 .. _qgiscondition:
 
 Conditional branch
@@ -723,3 +783,13 @@ Python code
 .. include:: ../algs_include.rst
   :start-after: **algorithm_code_section**
   :end-before: **end_algorithm_code_section**
+
+
+.. Substitutions definitions - AVOID EDITING PAST THIS LINE
+   This will be automatically updated by the find_set_subst.py script.
+   If you need to create a new substitution manually,
+   please add it also to the substitutions.txt file in the
+   source folder.
+
+.. |334| replace:: ``NEW in 3.34``
+