Add description of `functools.partial` in Executors documentation

[jordandsullivan]

Discussed in #2326

^{Originally posted by jordandsullivan April 25, 2024}
In a fair number of example notebooks and tests, the utility functools.partial is used to creator an executor function with fewer arguments specific arguments set to particular values. e.g. https://github.com/unitaryfund/mitiq/blob/main/docs/source/examples/ddd_tutorial.md?plain=1#L182.

Given how common a use case this is (i.e. specifying kwargs for an executor but still running it as a function), I suggest we add a short description and example of this in our Executors documentation page.