Updated docs for randint operator

[ChaiBapchya]

Description

Updated the docs to reflect randint operator
Added for both NDArray and Symbol API (RandomNumberGenerator API)

[aaronmarkham]

Please try to keep the API references in categorical then alphabetical order.
Also, a description is needed in your PR's main comment.

Once your PR gets through CI, use the preview link template as a guide for providing links to each update so it is easier to review.

For example, http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-13541/1/api/python/ndarray/ndarray.html#random-sampling

[anirudh2290]

You can test the docs here: http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-13541/1 . Looks like the docs for mx.nd.random.randint are not generated. Please check if randint is added to the __all__ list.

[aaronmarkham]

Here's the current output at: http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-13541/3/api/python/ndarray/ndarray.html#random-sampling

First thing I'm noticing is that you probably could have it showing minus mxnet.ndarray so each entry is like random.x. Note how the other sections are like this.

I see that mxnet.random.seed is different and not part of ndarray, so it make sense to have the full name.

Then within this Random Sampling category, the entries should be alphabetical. Rooting around in a long list for the function you want is difficult when it isn't alphabetical.