DOC: Overhaul - new skin, new deployment & multiversion archival

[oesteban]

This PR corresponds to the manual update to the gh-pages I've done (and currently available at https://nipreps.org/niworkflows).

Inherently, I'm proposing the following changes:

• Only the tip of each MAJOR.MINOR version series is archived. If we roughly follow our own conventions for the patch releases, this should not be a problem. I think the gains in clarity (without uneven numbers of patch releases clouding the choice) and space (we only need to store one version per series) outweigh any caveat of not having access to older patch release docs.
• Drop the sphinxcontrib-versioning package - we just need to be sure we do not ever wipe gh-pages out. Now, the old versions are archived and only overwritten if a new patch release of that series is issued. The responsibility of these updates is moved to GH Actions (work in progress).
• For the version chooser widget to work with future versions that will be uploaded, it has been modified to now be dynamic and read an index of versions from a JSON file (versions.json).
• To minimize the necessary dependencies, nilearn, PyBIDS,nitransforms, and other packages have been moved to a lazy import fashion. Although autodoc has the capability of "mocking" imports, we are using Nipype's sphinx extension to generate the documentation of Nipype Interfaces. That extension (which I wrote), does not mock the imports. Considering the memory issues we generally have, it might be beneficial to have these imports hidden, anyways.

I'm also changing the skin of the documentation with the furo theme. It felt much cleaner than the RTD skin, and turned out to be easy to customize to include the version switch right below the project name.

I'm tagging @rwblair as a reviewer because he had quite a bit of time struggling with the sphinxcontrib-versioning package and the overall setup - his feedback could be very relevant, and at the very least, he might be interested too in the solution.

EDIT: added comment about imports

[effigies]

The changes listed in your top post make sense to me. Looks like you're actively working on this, so please @ me when you want feedback.

[oesteban]

@effigies - I'm done. The deployment of new tags seems to be working (https://nipreps.org/niworkflows/0.0/ - I'll clean this up manually when this PR is merged). That was the main focus for my latest commits. We'll see how it fails when it comes to update master after merging.

[effigies]

Posted this on sdcflows by accident:

This is going to make it trickier to rerender older versions at need. Or rather, it will create a watershed where we do one thing to render before and another thing to render after. Even if we can't get a single "correct" result now, I would suggest that we leave this rule unchanged and create a new rule like htmlbranch.

[oesteban]

Replied at the site of the accident ;)

[oesteban]

Okay, I would need to unlock myself from this one - so if I didn't really address the open question about the Makefile @effigies, let's go back to it after merge. I really hope that's fine :)