Enable RTD builds PR support

[yarikoptic]

RTD people have an experimental feature of providing builds for PRs. See e.g. con/open-brain-consent#50 where it was already enabled and tested.

I do not have admin privileges for RTD for this project, so the most I could do is to pester RTD people to enable the feature for this repository, which I was told they have done! So it is now for people with the super powers to actually make it work. Should be quite straightforward, see https://docs.readthedocs.io/en/latest/guides/autobuild-docs-for-pull-requests.html for the documentation and common gotchas. In my case it was the need "to disconnect and reconnect to GitHub service" as now described in that document.

[yarikoptic]

FWIW, BIDS specification now has it enabled too: bids-standard/bids-specification#484

[adswa]

Done, thx much!