[REVIEW] Improve Documentation Examples and Source Linking

[mdemoret-nv]

Adding some features from sklearn's documentation into cuml. This includes the ability to toggle the output of examples directly in the HTML documentation and adding source links for each class/method.

In order to get the source linking to work, Cython needed to be compiled with binding=True for the compiler directives. See this link for an explanation: https://opendreamkit.org/2017/06/09/CythonSphinx/

Creating this WIP PR to evaluate any performance regressions since binding=True changes how Cython calls pyx functions.

@dantegd Would like to discuss the Cython build process to determine the best way to include binding=True in the current system. I took some inspiration from this Cython issue and its working relatively well.. Would appreciate any feedback since it would be a large change to the build system.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[mdemoret-nv]

PR is ready to review. Depends on PR #2638 which has been merged into this branch already (so some of the diff files will be due to that PR).

Be sure to look at cuml.metrics.pairwise_distances.pairwise_distances example to see the new toggle button. Many of the examples still use the old style which would be cleaned up in issue #2415.

[mdemoret-nv]

@JohnZed This should be good for review pending CI. I added the copyright messages as we discussed to the 3 new files. Not sure if we should add the NV copyright as well. Let me know and I can make the change quickly.

[JohnZed]

Looks great! We should get a quick @rapidsai/ops-codeowners signoff as well. It wasn't triggered by these files, but the ops team drives much of the doc publishing, so let's loop them in.

[mdemoret-nv]

rerun tests

[ajschmidt8]

confirmed source code link locally. awesome change! 🔥 thanks for the heads up @JohnZed