Test PR: Use dirhtml builder to see if #1407 works

[benjaoming]

Testing #1407 with dirhtml builder enabled

[benjaoming]

The test indicates that .html is trimmed. I'm not sure if the Canonical URL on PR builds is supposed to point to the stable branch, but it seems like a good idea to avoid search indexing PR builds.

[Blendify]

I am noticing that sphinx itself would have the same issue here: https://github.com/sphinx-doc/sphinx/blob/master/sphinx/themes/basic/layout.html#L132

Probably best to use a mixture of https://www.sphinx-doc.org/en/master/development/templating.html#file_suffix
and https://www.sphinx-doc.org/en/master/development/templating.html#pagename

[benjaoming]

@Blendify responded here: #1407 (comment)