[WIP] Adding `qml.OutPoly' (#6320)

To do:
- [x] Final review
- [x] Add thumbnail
- [x] Add changeling

In this PR we add `qml.OutPoly`. An operator that allows to apply
arbitrary polynomials on quantum registers.

Example where we want to create the operator:
$$U|x\rangle |y\rangle |0\rangle = |x\rangle |y\rangle |x^2 + y\rangle$$

```python

import pennylane as qml

x_wires = [0, 1, 2]
y_wires = [3, 4, 5]

output_wires = [6, 7, 8]


def f(x, y):
    return x ** 2 + y


@qml.qnode(qml.device("default.qubit", shots = 1))
def circuit():
    # loading values for x and y
    qml.BasisEmbedding(2, wires=x_wires)
    qml.BasisEmbedding(2 , wires=y_wires)

    # applying the polynomial
    qml.OutPoly(f,
                input_registers = [x_wires, y_wires],
                output_wires=output_wires)

    return qml.sample(wires=output_wires)

print(circuit())

```

**Context:**
This operator is born of the quantum arithmetic project 


**Benefits:**

complete flexibility for the user to create new arithmetic operators


[[sc-72629](https://app.shortcut.com/xanaduai/story/72629/support-polynomial-arithmetic)]

---------

Co-authored-by: Utkarsh <utkarshazad98@gmail.com>
Co-authored-by: Will <wmaxwell90@gmail.com>
Co-authored-by: Isaac De Vlugt <34751083+isaacdevlugt@users.noreply.github.com>
Co-authored-by: soranjh <40344468+soranjh@users.noreply.github.com>

Author: 5 people

doc/_static/templates/arithmetic/outpoly.png

@@ -158,6 +158,10 @@ as addition, multiplication and exponentiation.
     :description: :doc:`IntegerComparator <../code/api/pennylane.IntegerComparator>`
     :figure: _static/templates/arithmetic/integercomparator.png
 
+.. gallery-item::
+    :description: :doc:`OutPoly <../code/api/pennylane.OutPoly>`
+    :figure: _static/templates/arithmetic/outpoly.png
+
 
 .. raw:: html
 

doc/introduction/templates.rst

@@ -79,6 +79,9 @@
 
 <h4>Calculating Polynomials 🔢</h4>
 
+* `qml.OutPoly` template is added to implement polynomial arithmetic.
+  [(#6320)](https://github.com/PennyLaneAI/pennylane/pull/6320)
+
 <h4>Readout Noise 📠</h4>
 
 * Support for applying readout errors to a quantum circuit has been added via the ``NoiseModel`` class

doc/releases/changelog-dev.md

@@ -51,3 +51,4 @@
 from .out_multiplier import OutMultiplier
 from .out_adder import OutAdder
 from .mod_exp import ModExp
+from .out_poly import OutPoly