Daily rc sync to master (#6200)

Automatic sync from the release candidate to master during a feature
freeze.

---------

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Co-authored-by: Christina Lee <christina@xanadu.ai>
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doc/code/qml_spin.rst

@@ -0,0 +1,15 @@
+qml.spin
+=========
+
+Overview
+--------
+
+This module contains functions and classes for creating and manipulating Hamiltonians for the spin models.
+
+.. currentmodule:: pennylane.spin
+
+.. automodapi:: pennylane.spin
+    :no-heading:
+    :no-main-docstr:
+    :skip: Lattice
+    :include-all-objects:

doc/development/deprecations.rst

@@ -16,7 +16,7 @@ Pending deprecations
   - Will be removed in v0.39
 
 * The logic for internally switching a device for a different backpropagation
-  compatible device is now deprecated, as it was in place for the deprecated `default.qubit.legacy`.
+  compatible device is now deprecated, as it was in place for the deprecated ``default.qubit.legacy``.
 
   - Deprecated in v0.38
   - Will be removed in v0.39  

doc/index.rst

@@ -216,7 +216,8 @@ PennyLane is **free** and **open source**, released under the Apache License, Ve
    code/qml_qaoa
    code/qml_qchem
    code/qml_qnn
-
+   code/qml_spin
+
 .. toctree::
    :maxdepth: 1
    :caption: Internals

doc/introduction/templates.rst

@@ -124,6 +124,45 @@ state preparation is typically used as the first operation.
 
 .. _intro_ref_temp_subroutines:
 
+Arithmetic templates
+--------------------
+
+Quantum arithmetic templates enable in-place and out-place modular operations such 
+as addition, multiplication and exponentiation.
+
+.. gallery-item::
+    :description: :doc:`PhaseAdder <../code/api/pennylane.PhaseAdder>`
+    :figure: _static/templates/arithmetic/phaseadder.png
+
+.. gallery-item::
+    :description: :doc:`Adder <../code/api/pennylane.Adder>`
+    :figure: _static/templates/arithmetic/adder.png
+
+.. gallery-item::
+    :description: :doc:`OutAdder <../code/api/pennylane.OutAdder>`
+    :figure: _static/templates/arithmetic/outadder.png
+
+.. gallery-item::
+    :description: :doc:`Multiplier <../code/api/pennylane.Multiplier>`
+    :figure: _static/templates/arithmetic/multiplier.png
+
+.. gallery-item::
+    :description: :doc:`OutMultiplier <../code/api/pennylane.OutMultiplier>`
+    :figure: _static/templates/arithmetic/outmultiplier.png
+
+.. gallery-item::
+    :description: :doc:`ModExp <../code/api/pennylane.ModExp>`
+    :figure: _static/templates/arithmetic/modexp.png
+
+.. gallery-item::
+    :description: :doc:`IntegerComparator <../code/api/pennylane.IntegerComparator>`
+    :figure: _static/templates/arithmetic/integercomparator.png
+
+
+.. raw:: html
+
+        <div style='clear:both'></div>
+
 Quantum Chemistry templates
 ---------------------------