Enh rel canonical

This adds **rel=canonical** to all the pages in stable..
turns out it was a lot simpler than I thought. 

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Just to be clear, this will add a (for example)

`<link rel="canonical" href="http://scikit-learn.org/stable/modules/ensemble.html">`

to that page
and will do so for every auto-generated page on the **stable** docs.
This will appear in the `<head>` tag of each page. I Tested it on an online build too.

Apparently it will take a bit of time before we see any effect, as Googles crawlers need to update this.

For more info on what this is for, see [this article](https://support.google.com/webmasters/answer/139394?hl=en)