[REVIEW] Improve Python Docs with Default Role

[mdemoret-nv]

This PR sets the default type of "interpolated text" in sphinx to :py:obj:. This is very useful for us since we frequently use a single backtick in our python documentation to refer to another python object.

Currently, the docstring:

`cuml.datasets.make_blobs`

Would generate (italicized, variable spaced):

This PR changes it to (bold, mono spaced):

The added benefit here is if the interpolated text is found in the index, it will link to that section. So in the above example, clicking on cuml.datasets.make_blobs will take you to the function documentation.

Finally, this PR adds a new type of interpolated text role: :py: . This should be used for inline python code. For example, the following code:

 * `cuml.cuml.datasets.make_blobs` for references do objects (functions, classes, modules, etc.)
 * :py:`import cupy as cp` for inline python code
 * ``import cupy as cp`` for literal code

will generate:

I also looked for a few examples to replace to help seed usage of these new options. Updating every location would be very time consuming and is best done over time.

[mdemoret-nv]

Note: This PR doesn't significantly change the existing functionality. Only single backticks will be changed to better align with how developers have been using them. Double backticks for literal code are unchanged, and the new :py: inline code is new.

[dantegd]

lgtm, very nice imporovement. @mdemoret-nv do you think it'd be a good idea to open an issue about adopting the new py feature uniformly around?

[mdemoret-nv]

lgtm, very nice imporovement. @mdemoret-nv do you think it'd be a good idea to open an issue about adopting the new py feature uniformly around?

We can. It would be a very tedious process for one person but would have a nice effect on the docs. Could be a team effort. @JohnZed what do you think?

[JohnZed]

Agreed on opening an issue and adding to the Technical Debt project. We can also move over time as people update various submodules, they can update docstrings accordingly.
BTW, this change looks great

[JohnZed]

@gpucibot merge

[mdemoret-nv]

Agreed on opening an issue and adding to the Technical Debt project. We can also move over time as people update various submodules, they can update docstrings accordingly.
BTW, this change looks great

Done: #3451