Add support for `xdoctest`

[prady0t]

Description

Now that #3857 is merged, we can add support for xdoctest for testing doctests. Xdoctest is a pytest equivalent to python's build-in doctest. See more : https://github.com/pybamm-team/PyBaMM/issues/3617#issuecomment-1854702640

Related to : #3617

[agriyakhetarpal]

Hi, @prady0t – are you already on this? Let us know if you are, and one of us can assign you to it

[prady0t]

Yes please assign this to me