MAINT: Add website builds for easier PR review

[effigies]

Related to ongoing discussion in #76, it would be good to enable PR-associated builds to simplify reviews.

The approach used in bids-standard/bids-specification#315 seems pretty straightforward. As an organization owner, I will go ahead and enable CircleCI and the artifacts redirector on this repository. If somebody has the time and inclination to put together a CircleCI config, please comment and let us know.

If nobody has time before then, I should have some on Monday.

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This isn't to short-circuit discussion about netlify or other options. I vaguely remember not finding it easy to get netlify going, but I can't remember why.

I also recently saw nipreps/dmriprep#52, which suggests that perhaps there's a way to use GitHub actions to build docs on PRs. It hasn't been merged, so I don't know how viewing them would look.