Add links to code in docstrings

[jeffreykennethli]

In our readthedocs, our docstrings often refer to other parts of the codebase, for example in read_csv_glob there is a reference to read_csv: link.

In pandas, the docstring references contain links to other parts of the codebase. It looks like this is accomplished with rst external links, see example: https://github.com/pandas-dev/pandas/blob/945c9ed766a61c7d2c0a7cbb251b6edebf9cb7d5/pandas/io/parsers/readers.py#L56-L64 and explanation of external links with sphinx: https://sublime-and-sphinx-guide.readthedocs.io/en/latest/references.html#links-to-external-web-pages

I think this would be worth considering adding for Modin for ease of navigating the readthedocs, although it may come at the cost of making the raw docstrings (in github) harder to read.

[jeffreykennethli]

We should do something like this to make our readthedocs documentation richer: https://www.sphinx-doc.org/en/master/tutorial/describing-code.html?highlight=cross-references#cross-referencing-python-objects