Transfer repository to github.com/healthysustainablecities

[carlhiggs]

We previously discussed and voted on consolidating our organisation names to be consistent with our Global Observatory of Healthy and Sustainabel Cities' URL https://healthysustainablecities.org (discussion here: https://github.com/global-healthy-liveable-cities/global-indicators/discussions/353).

We set up our new organisation,
https://github.com/healthysustainablecities

and our documentation website
https://healthysustainablecities.github.io/

But we have taken our time to migrate the repository itself.

The time is now! @gboeing clarified the correct way to do this recently via a repo transfer, and this issue is to formalise our intent to do this.

Once the repository is transferred, we'll create a fork here that we can archive.