Due Oct 2025- Carbon offsets

This is a spin-off from #16. In that thread we were discussing the broad goal of achieving carbon-neutrality. A lot of the discussion there was on travel, but CI was also raised as a major carbon source of the project.

That said, last comment there was in January, and since then... our travel-related carbon footprint has fallen dramatically as a project! Not for the best of reasons perhaps but there it is anyway. So I'm creating this issue to focus specifically on the specific topic of offsetting our CI.

In https://github.com/astropy/astropy-project/issues/16#issuecomment-565225890 I did a back-of-the-envelope estimate of our current usage. My best guess there was 5 g of CO2 per CI build, and a total to-date of the core package *only* (which is probably the largest but not the only significant CI load) at 1.5 metric tons.

What I have *not* yet done is estimated things like `astroquery` server impact, etc. Not all of that is entirely Astropy's "fault", but we could certainly make some ballpark estimates.

So my proposal is that we work out a permanent (or at least permanent-for-as-long-as-we-have-money) mechanism we can use to do carbon offsetting for this work. As discussed in #16, it also makes sense to limit our footprint in the first place, but that comes with lots of tradeoffs that require discussion, whereas it seemed to me there was a clear consensus that offsets are the least we can do.

@astrofrog gave a list of resources to consider here: https://github.com/astropy/astropy-project/issues/16#issuecomment-566977382 for carbon offsets. I haven't yet had a chance to absorb them, but I'm thinking if we pick from something those suggestions lead to that might be the easiest approach?

Start by reading issue #16, especially the linked carbon-footprint estimate and the carbon-offset resources from @astrofrog. Review the existing CI and related-project estimates described here, then clarify the scope and funding mechanism. Done means the project has an agreed, sustainable approach to offsetting its CI footprint.

Written by the indexing model from the issue text.