Fix canonical URLs with dirhtml builder

[justinludwig]

As suggested by @ericholscher in #285, using the logic from the
readthedocs-sphinx-ext package.

Currently if you set the canonical_url theme option (for example, to
https://example.com/), a canonical link like the following will be
generated for source files like foo/bar.rst:

<link rel="canonical" href="https://example.com/foo/bar.html"/>

This is good for the html builder, but doesn't match the output of the
dirhtml builder. This commit changes canonical link generation like
the above example to the following when the dirhtml builder is used:

<link rel="canonical" href="https://example.com/foo/bar/"/>

Canonical link generation remains the same as before when the dirhtml
builder is not used.

[Blendify]

I would like to see this change added to sphinx itself first: https://github.com/sphinx-doc/sphinx/blob/master/sphinx/themes/basic/layout.html

This way we can inherit the html from sphinx.

[stsewd]

Thanks for the contribution, this is in sphinx itself now https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_baseurl, we should deprecate the canonical_url option in favor of html_baseurl https://github.com/sphinx-doc/sphinx/blob/e886cc21710e23bc53af83d011131828b1020848/sphinx/themes/basic/layout.html#L135-L137

[stsewd]

#1003