Add sklearn and random cut forest to sdk docs

[eslesar-aws]

Issue #, if available:

Description of changes:

I added an sklearn rst file in the doc directory, and added a corresponding section to the index.rst file to include it in the doc project. Also added randomcutforest to the list of 1st-party algorithms section in index.rst.

I have built the sphinx project locally and confirmed that the build looks correct.

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[yangaws]

LGTM

There're still several 1p missing in the doc. But we can add it in new PRs.

[laurenyu]

this also needs a corresponding file. or just remove the added line because earlier today I did try to make files for all the missing 1P algos in #575

[eslesar-aws]

@laurenyu I didn't see #575 ...this PR should be closed in favor of that one.