Deprecate canonical_url in favor of html_baseurl

[stsewd]

Rely on html_baseurl for the canonical url instead. The canonical url is already set in the basic theme

https://github.com/sphinx-doc/sphinx/blob/4e9d1553968653df6699b20afc22b73eb3280185/sphinx/themes/basic/layout.html#L130-L132

That takes into account pages using the dirhtml builder instead of always adding the html extension. This was introduced in sphinx 1.8.