Implementing Exact Tensor Network method for default.tensor (#5786)

**Context:** Currently, the new `default.tensor` device can only
simulate quantum circuits using the Matrix Product State (MPS) method,
based on `quimb`'s `CircuitMPS` class.

**Description of the Change:** With this PR, the device also supports
the Exact Tensor Network method based on `quimb`'s `Circuit` class. This
completes the implementation of the `method` keyword argument, which can
be set to either ``mps`` or ``tn``. For this quarter, this is expected
to be the last feature for the `default.tensor` device (except for
refinements, bug fixes, and performance improvements).

**Benefits:** A new simulation method is implemented for this device,
enabling new functionalities for users.

**Possible Drawbacks:** This PR just implements an additional method,
but it does not modify the existing functionalities (except for some
minor improvements and clarifications in the documentation).

**Related GitHub Issues:** None.

**Related Shortcut Stories:** 
[sc-60805] 
[sc-64011]
[sc-64284]

doc/releases/changelog-dev.md

@@ -4,6 +4,9 @@
 
 <h3>New features since last release</h3>
 
+* The `default.tensor` device now supports the `tn` method to simulate quantum circuits using exact tensor networks.
+  [(#5786)](https://github.com/PennyLaneAI/pennylane/pull/5786)
+
 * QROM template is added. This template allows you to enter classic data in the form of bitstrings.
   [(#5688)](https://github.com/PennyLaneAI/pennylane/pull/5688)
 
@@ -46,7 +49,7 @@
     to execute once for each shot. If using `qml.jit` with the Catalyst compiler, `mcm_method="single-branch-statistics"`
     is also available. Using this method, a single branch of the execution tree will be randomly explored.
 
-* The `default.tensor` device is introduced to perform tensor network simulation of a quantum circuit.
+* The `default.tensor` device is introduced to perform tensor network simulations of quantum circuits using the `mps` (Matrix Product State) method.
   [(#5699)](https://github.com/PennyLaneAI/pennylane/pull/5699)
 
 * A new `qml.noise` module which contains utility functions for building `NoiseModels`.