Add paper reference to OBP guide page

[jenglick]

Include references to the OBP paper in the overview guide and a corresponding reference section at the bottom of page.

I suggest the following places:

At the end of the second sentence of the overview guide:

Operator backpropagation (OBP) is a technique to reduce circuit depth by trimming operations from its end at the cost of more operator measurements. There are a number of ways in which operator backpropagation can be performed, and this package uses a method based on Clifford perturbation theory [1].

Add a new sentence to the start of the Theoretical Background section:

The OBP procedure implemented in this package is described in detail in [1]. When using the Estimator primitive, the output of a quantum workload is the estimation of one or more expectation...

[1] https://arxiv.org/abs/2502.01897

cc @BryceFuller